Mercedes-Benz Ponton
Tire Guide Discussion II

I. INTRODUCTION

Finding the best tire for Mercedes-Benz Pontons has been the subject of debate for a long time. Terry Morgan has provided an extremely useful tire guide and discussion. See: http://www.mbzponton.org/valueadded/technical/tireguide.htm

He starts from the premise that the modern size 185/80R13 is clearly the most appropriate current radial replacement for both sizes of the original 13-inch Ponton tires. Well, yes and no.

Terry and I had, admittedly, somewhat different goals. His apparently was to identify the most suitable radial tire for reasons of safety. Mine was to find a modern radial tire available today that most closely corresponds to the measurement characteristics of the original tires. My prioritized objectives were (1) to keep the original 13-inch wheels, (2) to come as close as possible to the revolutions per mile (RPM) of the original tires, and (3) to have the spare tire slip easily into the spare tire well in the trunk.

I was also looking for comparable aspect ratios, and that involves tire diameter and section width. Additional considerations included tire height and aesthetics inside the fender well and with respect to the overall appearance lines of the vehicle. Cost too was important.

II. SUMMARY CONCLUSION

Conventional wisdom generally has been that the modern 185/80R13 size is the best radial tire equivalent for the original tires on all Ponton models. I found, contrarily, that one should consider other size categories as well. The following table presents the original tire and, according to my findings, the modern equivalents:

Table 1 / Original Tires and Modern Equivalents:

The overlaps occur because of the measured differences among modern tire brands and because of physical differences between lines within the same manufacturer. So, what modern tire size should I buy, one rightfully asks? How should I interpret the table? Give a clear, unequivocal answer and cut the hedging!

The answer is a straight-from-the-shoulder evasion: It depends. On what? If you can handle the prolixity, read the whole piece below. If not, the above table will do. Physical measurements may be necessary. Suffice it to say that I bought a satisfactory tire for my 220S that physically measured 181/83R13 and calculated to 811 RPM. Suffice it to say further that I might not have made the same purchase now that I have investigated the matter more.

III. BEWARE

In reading the discussion below, one needs to recognize that the results reflect an inexact reality. Although we start from some ostensibly known data, inconsistencies or errors in published information, rounding errors, differences among manufacturers, differences even with different production runs can all introduce complications. Not to mention the possibility of my own analytical mistakes.

IV. BASIC INFORMATION

Generally the characteristics of the tires on our Pontons were:

Table 2 / General Characteristics of Ponton Tires:

Original Size	Original Section Width (in / mm) [1]	Original RPM	Original Aspect
6.40-13	6.4 / 162.6	857	82
6.70-13	6.7 / 170.2	834	83
6.40-15	6.4 / 162.6	786	83

This means 13-inch wheels and either 6.4 inches or 6.7 inches section width. At the time section widths were expressed in inches.

Expressed in modern terms the 6.40-13 tire would be 163/82-13. The 6.70-13 tire would be 170/83-13. The 6.40-15 tire would be 163/83-15.

At some early point Mercedes-Benz officially substituted the size 7.25-13 tire for the original 6.70-13 size on the 220S and 220SE. There was an optional 6.40-15 tire as well, available on all Pontons. For commercial purposes an optional 6.70-14 tire was available.

The article will talk about modern radial size categories. The 185/80R13 category, for example, means: 185 mm maximum section width, 80 minimum aspect ratio, 13 inches wheel rim. For a good explication of what all this means see: http://www.tirerack.com/tires/tiretech/general/size.jsp. Application of these designations for Ponton purposes is seen below.

V. HISTORY

The Great Tire Quest is not at all new. In one of the Star Reprint series (No. 2 @ 39) it states that modern radial sizes 185/80-13 or 175/80R13 are the most suitable. In an article on p. 11 in the November/December 1995 Star the late Frank King states that for the 6.70-13 tire the best equivalent is 195/70R13, and for the 6.40-13 tire the equivalent is 185/70R13. (His discussions about revolutions per mile yield mathematically questionable results.) Note also the considerable treatment of this issue by the 190SL group: http://www.190slgroup.com Opinions thus vary widely.

VI. RADIUS, DIAMETER, CIRCUMFERENCE, AND REVOLUTIONS PER MILE (RPM)

The departure point is the tire sections, 40-0/2 and 40-0/3, of the Type 190 Service Manual. One should have it accessible when reading the discussion that follows.

See the Type 190 Service Manual's discussion of "effective dynamic radius." It yields essential information:

"The effective dynamic radius is determined by dividing the distance traveled per revolution of the wheel at a speed of 60 km/h by 2 Π.[2]  The tire pressure must be carrying the maximum permissible load and must be inflated to the specified air pressure."

This is approximately 35 MPH. It means that the nominal reference point is at moderate speed, fully inflated, and at the vehicle's maximum permitted load. The significance of this is apparent below.

6.40-13

See what happens with the original 6.40-13 tire. Section 40-0/3 of the Type 190 Service Manual lists 299 mm as the effective dynamic radius at 35 MPH. Double that to obtain the effective dynamic diameter: 598 mm. Convert this to inches: 598 ÷ 25.4 = 23.54. Thus the calculated diameter of this tire at moderate speed is 23.54 inches. How does this get to tire RPM? Mathematical whizzes know that circumference equals pi times diameter: C = Π d. So, multiply 3.1416 times 23.54 to get 73.95 inches diameter. Thus the original tire was nominally just shy of 74 inches in circumference. It would travel this number of inches in a single revolution at 35 MPH. One mile contains 63,360 inches (5,280 x 12). Dividing 63,360 by 73.95 yields a calculated 857 RPM figure. (63,360 ÷ 73.95 = 856.8.) Thus we see that the original tire revolved about 857 times each mile. That was Mercedes-Benz' established norm for the 6.40-13 tire.[3]

6.70-13

The Type 190 Service Manual states the effective dynamic radius at 307 mm for the 6.70-13 tire. Following the above calculation process for the 6.40-13 tire, this yields a dynamic diameter of 24.17 inches. That results in a calculation of 75.93 inches circumference which calculates to approximately 834 RPM at moderate speed. Thus the original 6.70-13 tire revolved about 834 times each mile, according to Mercedes-Benz norms.

7.25-13

The 6.70-13 tire continued into the 1960s as original fitment on some fintail models. Curiously, the 1960 Technical Data Manual (TDM) reprint shows the size 7.25-13 tire as having superceded the 6.70-13 tire. Specifically for the 220S and 220SE Pontons the TDM states:[4] "According to recent standardization, the old tire size 6.70-13 has been changed into 7.25-13." That this appeared in the 1960 TDM reprint was a puzzlement.[5]

Important, though, is the fact that the 7.25-13 tire did become officially standardized for the 220S and 220SE Pontons, probably in the mid-1960s. The 7.25 means the section width in inches. It converts to 184 mm. (Hence a modern 185 series radial becomes very much "officially" suitable.) Unfortunately neither the TDM nor the Service Manual provides any technical specifications like effective dynamic radius for the 7.25-13 tire. One cannot, therefore, determine RPM according to Mercedes-Benz norms from published Mercedes-Benz data. I did not unearth any technical data anywhere else either. If anyone has hard information, please let me know so I can make necessary changes.

6.70-14

I was unable to locate any data about the optional 6.70-14 tire. If anyone has any hard information, I will gladly include it here.

6.40-15

By calculation based on the data in the Type 190 Service Manual the optional 6.40-15 tire revolved about 786 times each mile at moderate speed, according to Mercedes-Benz published norms. I did not have one available for "ground truthing."

VII. ASPECT RATIO

In period pictures some tires of the 1950s look quite tall on the cars. Wide white wall tires originally supplied to some Pontons exacerbated this appearance. An old Firestone technical manual states that the Ponton tires had aspect ratios on the order of 93 to 95. A correspondent from Britain said the same thing about Pontons delivered in the United Kingdom. I discovered inconsistent and conflicting information when I explored the aspect ratio issue. As is my wont, I began to dig.

6.40-13

The aspect is the ratio of the width of the tire at the widest point on the casing (not the tread footprint) to the distance from the wheel hole to the top of the tire. Subtract the wheel rim size (13 inches)[6] from the diameter and divide by 2. For the 6.40-13 tire this is: 23.54 - 13 ÷ 2 = 5.27. That is the distance from the edge of the rim hole to the top of the tire on a plane with the center point of the tread. The stated section width is 6.40 inches.[7] Thus, 5.27 ÷ 6.4 = 0.823. Eureka! The nominal aspect ratio of the original 6.40-13 tire was 82.

6.70-13

Using this same process, the ratio for the original 6.70-13 tire was 0.834. The nominal aspect ratio of the original 6.70-13 tire was 83.

6.40-15

Again following the same process based on the data in the Service Manual, except using 15 inches for the wheel, the ratio for the optional 6.40-15 tire was 0.833. The nominal aspect ratio of this original tire thus was 83. For that to be the case, however, the tire had to be taller than the 6.40-13 tire by slightly over two inches, and it had to be taller than the 6.70-13 tire by about 1.5 inches. That makes sense because 15-inch tires were fitted for markets requiring higher ground clearance.

7.25-13

Recall the 7.25-13 tire indicated in the 1960 TDM reprint. Its calculated aspect ratio is 77. That assumes its circumference was the same as the 6.70-13 tire's circumference.[8] To reach an aspect ratio of 83 the 7.25-inch tire would have been nine percent taller than the 6.70-13 tire it replaced. That, in turn, would have had a nine-percent effect on the RPM and the speedometer/odometer. Compared to the 6.70-13 tire, the 7.25-13 successor tire either (1) was taller or (2) had a lesser aspect ratio. Unfortunately there is no information about the nominal data for the 7.25-13 successor tire, so I cannot determine which was the case.

Observation

The 82 and 83 original aspect ratios appear to make sense when carefully scrutinizing original Ponton photographs. Given the size and aesthetic lines of the cars, the tires do not appear exceptionally tall, especially when black wall. This is particularly striking on the 190SL and the 220S/SE coupés. Their style lines seem to do this. The very wide white walls of the period lend an illusion of tires being quite tall and filling up the fender well. Look at the pictures below:[9]

Photo 1:

Those tires do look tall!  That seems to be an illusory effect.

Photo 2:

Factory sales brochure showing a Type 190b with wide white wall tires.

VIII. MY CONSIDERATIONS

I elected to retain the original 13-inch wheel size on my 1959 Type 220S cabriolet. I knew that some Ponton owners, notably those of 190SLs, have converted to 14-inch tires. Apparently there is a 14-inch tire size that matches the original Ponton 6.40-13 tire RPM almost exactly. Sometimes 190SL owners want to reduce the engine RPM at high speed, and a 14-inch tire helps do that. Some Ponton sedan owners have reported success and better handling by converting to 14-inch wheels. I confined my investigation to 13-inch tires.

I wanted the spare tire to be the same size as the four on the ground. Also I preferred that the spare fit easily into the spare tire well in the trunk without stuffing. My existing spare was an old 185/80R13 Firestone cheapie radial that I had to force into the well. I did not want the spare lying flat on the trunk floor.

I also wanted to be as close to the original aspect ratio as possible. That focused me on 13-inch radials with a nominal 80 aspect ratio. By definition 14-inch tires that fit on a Ponton would have had a lower aspect ratio to stay within the RPM range of the original tires.

Finally, having a cabriolet I wanted white walls, wide if possible. My car was wearing Coker 650-13 crossply tires with 2.25-inch white walls. They did look good. I was willing, though, to compromise on the white wall width if necessary. That proved to be so.

Cost was an important consideration. The Coker crossply tires on my car appeared to be in good condition, although they were quite old hence potentially dangerous. My choice was between getting a fifth Coker for the spare or converting all five to new radials. The Cokers steered very hard compared to modern radials, but that must have been similar to the way the original crossply tires behaved. How one forgets! A new Coker was well upwards of $100 plus tube, shipping, and mounting/balancing. Coker Tire had a couple of other wide white wall choices that were rather pricier. Commonly available radials can be much less expensive, but there is a wide price range among brands.

IX. THE SEARCH

I made a detailed search until I realized it was not getting me useful answers. Misinformation was cropping up. I have relegated a discussion of my search to Annex 1, below.

X. THINKING THINGS THROUGH

I finally concluded that I would not be able to make an informed, detailed choice among brands based on technical data published by the manufacturers. What other considerations would I have? Price would be important, of course, but so would length of tire life. What about quality, safety, and appearance?

At first blush we tend to think that our beloved Pontons should be equipped with only the very best, highest quality, and highest mileage tire. Put a cheapie on a Ponton, and a cabriolet to boot? Simply not done! I thought this through further. Tires become tired -pun intended- because either they wear out treadwise or they deteriorate and rot because of age and exposure. Most of us, me included, are unlikely to drive our Pontons enough miles to warrant a high-mileage tire. They will rot or deteriorate to dangerousness well before the tread wears out. I concluded I did not need a high-mileage tire and did not need to pay the associated cost.

Did I need a high-speed safety-type tire? Unlikely at the leisurely speed and intensity my Ponton goes. So I concluded I did not need to pay a premium for such a tire.

185/80R13

All this compelled me to look closely at the 185/80R13 Sears Guardsman III tire. It is made by Bridgestone. It has average temperature and traction grades and 35,000 miles tread wear. It was cheap, amply satisfying my cost considerations! I could buy five for a total of $83.00 plus tax, mounting, and balancing. I tolerated its half-inch-wide white wall.

How did it turn out?

How do the actual measurements of the Sears Guardsman III correspond to Sears' published data? How does this relate to my Ponton requirements? I inflated the tire to 30 pounds. Its circumference measured 78.1 inches.[10] That calculates to 24.86 inches diameter which was, as well, what I obtained measuring it physically lying on the floor. That calculates to 811 RPM. Recall that the original 6.70-13 tire rolled 834 RPM. The difference from the original RPM, therefore, is 2.8 percent. Theoretically the deviation at the speedometer/odometer, assuming the speedo/odo to be absolutely correct, was 2.8 percent.

There was an unexpected bonus. Mounted and inflated, the Guardsman III measured exactly 181 mm (7.13 in.) at the section width. It slipped into the spare tire well easily, with room to spare (pun noted).

Yet another bonus turned up serendipitously. The aspect ratio of the Guardsman III calculated to 83. Thus it is identical to the original 6.70-13 tire's aspect ratio.

I had bought a tire in the 185/80R13 nominal size category. By both measurement and calculation it is truly size 181/83R13. That is 10 mm wider at the section than the original 6.70-13 tire, but the aspect ratio is the same. (See the table in section IV above.)

The thin white wall is not at all bad looking. See the pictures:[11]

Photo 3:

Photo 4:

This is not my cabriolet. Nevertheless the photos contrast the thin white walls found on modern radials, including the Guardsman I bought, with the wider white walls currently available on very expensive specialty tires, discussed below. It comes down to a matter of preference tempered by price. One curiosity by today's standards: the Taylor book refers to the wider of the two white walls depicted above as a "narrow-band whitewall."

XI. SIZE 175/80R13

Because I own a 220S my initial focus was the original 6.70-13 tire. I proceeded initially from the (now questionable) assumption that the modern 185/80R13 radial was likely the most appropriate. Generally that proved to be satisfactory, especially when I came to realize that this size designation is only a general category. It is not a precise standard. The conclusion is not at all clearcut.

What about the size 175/80R13? It may be even more commonly available than the 185/80R13 size. Theoretically the 175-mm section width equates to 6.9 inches, but this is a category maximum. My experience with other tires, discussed above, suggests that tires in the 175-mm category will physically have section widths slightly less than 175 mm. That proved to be true.

I had access to a 175R/80R13 tire (cannot recall brand) mounted on a Ponton wheel rim. I measured its circumference and section width using the same instruments as I used on the Sears tire above. It measured to approximately 75.5 inches diameter. That calculates to 839 RPM. The section width measured 6.625 inches or slightly above 168 mm. This leads to a calculated aspect ratio of about 83.

To confirm this measurement I checked the Sears web site for Guardsman III tire in the 175/80R13 category. It lists a diameter of 24.0 inches. That calculates to a circumference of 75.40 inches and 840 RPM. Recall from fn. 10 that the published data for the Sears 185/80R13 Guardsman III differed very little from the measurements I took. I therefore assumed that would hold true for this size tire.

Target: 170/83-13 and 834 RPM

Recall that, expressed in today's terms, the original 6.70-13 tire was 170/83-13; its RPM figure was 834. I had to conclude that, according to the criteria of my search, the 175/80R13 tire size is, as a category, a much closer match to the original 6.70-13 tire than the 185/80R13 tire I bought. This tire measures to 840 RPM. That is very close, less than a one-percent margin of error. As I did not have a tire mounted on a rim, I could not measure the true section width. The true aspect ratio therefore is unknown, but it probably somewhere between 81 and 83.

XII. SIZE 165/80R13

A good case can be made that a modern 165/80R13 category is a good place to look for Pontons that originally wore 6.40-13 tires. 165 mm converts to 6.5 inches. My investigation led me to conclude that most tires in a nominal size category have slightly smaller section widths than the nominal; the nominal is in effect a maximum. Hence most 165/80R13 tires will be between 161 and 164 mm section width. Note that 6.4 inches converts to 162.5 mm, so a tire actually measuring 162 or 163 section width would be ideal. (Recall: expressed in today's terms the original 6.40-13 tire was 163/82-13.)

What about the circumference? Not having such a tire at hand on a Ponton rim, I looked at a Guardian on the Sears web site.[12] The Sears tire calculates to about 857 RPM. That is about identical to the nominal RPM of the original 6.40-13 tire as seen in Section IV above.

XIII. ALTERNATIVES

White walls

Recall that I forewent my wish for wide white walls. My search unearthed only three wide white wall tubeless radial tires in the 13-inch wheel size. They are all very expensive, and four or five will be required.

(1) From time to time Mercedes-Benz offers a 6.40-7.00SR13 tire for Pontons. When available, it is Mercedes-Benz part number Q-4-40-101b. It is made in Europe by Continental, apparently in batches on order by Mercedes-Benz. Some kind of exclusive arrangement must exist. My resourceful tire supplier tried every avenue to identify this tire from Continental sources but was unsuccessful. I was not able to obtain any technical data. I saw them on a 190SL, and they were handsome. I was unable to obtain any physical measurements. If anyone can supply them, I'll be happy to incorporate them.

(2) Diamond Back Classics offers radial tires in Ponton-relevant sizes. See http://www.widewhitewalltires.com. One can do his own calculations of true section width, RPM. and aspect from the published data. As discussed, though, keep in mind that these may differ somewhat from the results derived from physical measurements.

(3) Also, Coker is offering a BF Goodrich Silvertown radial tubeless tire suitable for at least some Pontons. See: http://www.coker.com/store/customer/home.php?cat=365. Again, the calculations and measurements can be made by anyone interested. Scott Gordon took the plunge and waxes eloquent. See: http://www.mbzponton.org/valueadded/technical/tireswww.htm These tires do look good! To my knowledge these tires are not available through regular BF Goodrich vendors. Let me know if you have contrary information.

Conclusion: If you absolutely, positively must have what today passes for wide white wall radials, be prepared to pay heavily. They begin at upwards of $125 each to which you must add shipping, tax, and mounting/balancing.

Black wall

Something approaching originality is the Michelin Vintage X-stop 725R13 tire available in the United States only through Coker Tire. It is a radial but requires a tube at additional cost. Expect to be financially eviscerated for four or five tires. See pages 16-17 of Coker's catalogue: http://www.coker.com/store/files/pdf/2004catalog.pdf

I did not have one to measure. One can make the necessary RPM and aspect calculations from Coker's data table. It is stated to be the same diameter -height- as the original 6.40-15 tire discussed above; that is, 1.5 to 2-plus inches taller than the standard 13-inch Ponton tire fitment. Frank Barrett, editor of the Star magazine, fit these to his 220SE sedan. He reports they resulted in no speedometer/odometer deviation from before their fitting to his car. They look quite authentic.

XIV. TUBELESS OR NOT?

The Type 190 Service Manual states that a tubeless tire option was available as original fitment. Apparently this was not a radial. The manual provides no further discussion or data. There was no indication that a different wheel rim was required. Frequently one encounters strong advice that, when converting from original-type crossply tires to modern tubeless radials, a special tube is needed. That adds significantly to cost.

Did I need to use a tube when putting modern radials on my original wheel rims? I thought about that. Knowing that tubeless tires were an original option without requiring a rim change, I mounted the new radials to the wheels without a tube. My rims were undamaged. So far, so good: no loss of air pressure. No signs of slow leaks at the bead.

XV. INEVITABLE INEXACTITUDES

Although precision is ideal, many variables exist. One may exacerbate another. Another may mitigate or offset. Individual physical examination is necessary. Many considerations come into play. For a partial discussion, see Annex 2, below.

XVI. SOME DISCOVERIES

• The 185 section width in the modern size 185/80R13 is a maximum. Tires in this category often measure rather less. I found tires physically measuring 181 and 184. Doubtless there are still others.
• The 80 aspect ratio is also nominal; it is in fact a minimum for the category. I found tires physically measuring 82 and 83 aspect ratio.
• A tire by a manufacturer in a size category may well physically measure differently from the category nominal.
• Different tire lines from the same manufacturer in the same nominal category may have still different physical measurements one from another.
• Tires from different manufacturers within the same nominal size category are likely physically to measure differently one from another.

XVII. CONCLUSION

In terms of RPM and section width, a modern 165/80R13 radial may provide a close equivalent to the original 6.40-13 tire, depending on the manufacturer and line. With other manufacturers the nominal size 175/80R13 may prove closer to the original RPM number.

For the 6.70-13 original tire a modern 175/80R13 appears to provide the closest match, with the same caveat. But with Mercedes-Benz early on having officially supplanted the 6.70 with the 7.25 tire, a modern 185/80R13 category tire also becomes quite appropriate.

There is no holy grail. There are too many dependent and independent variables at play. The only way to be certain of tire data is to take physical measurements. And then you will have to test the results in a measured mile run for the odometer and against a stop watch for the speedometer.

Notes:

1. Web page links are valid as of this writing. If they become invalid, a web search engine should recover what you need.
2. Acknowledgement goes to Walter Yin and Scott Gordon for critical review along the way. Their suggestions were very helpful and their time much appreciated. Mistakes are mine, not theirs. Jeff Miller, editor-publisher of www.mbzponton.org, suffered through the usual agonies in preparing my article for presentation on page. He has my fullest appreciation.

Disclaimers:

1. The usual disclaimers apply. Use at your own risk.

Annex 1 - The Search

There are several choices in the 185/80R13 category: Cooper, Sears, Toyo, Kelly, Firestone, BF Goodrich, Kumho, Sumitomo for some. The trick was how to compare their characteristics from the literature before buying something. At first I would take each tire brand's published data at face value. Then, thanks to Frank Barrett, I came to understand that there were many variations among manufacturers and that the technical data sometimes were not comparable or were internally inconsistent. Indeed, the size 185/80R13 itself was a nominal category - not the 13-inch wheel diameter but the 185 section width and the 80 aspect.

I will not compare all the tires in detail. That would overwhelm the reader. One can employ the methodology when looking at other brands. Let's examine the Toyo 800 Ultra 185/80R13. They are good quality tires generally. A number of Ponton (including 190SL owners) have bought them.

Toyo 800 Ultra

The Toyo data sheet says that overall diameter of the 185/80R13 tire is 24.7 inches. Does that mean off or on the rim inflated? If on and inflated, does it mean in motion or stationary? If in motion, at what speed? If mounted and inflated, does it mean on or off the car? If on the car, does it mean with the car at stated maximum load or with vehicle unloaded? The data sheet does not answer these questions.

The Toyo data states the section width to be 7.4 inches. That converts to 188 mm. Is the 7.4 figure meant on the car stationary or in motion? At what speed? Again, this is unspecified.

Then Toyo says the static loaded radius is 11.1 inches. Does that mean on the car with the car at maximum stated load or the car at curb weight? What is the usefulness of this figure? Ground clearance or something else? Why is the word "loaded" used with respect to radius but not diameter? Why is the word "static" used for radius but not diameter? There is no explanation on the Toyo data sheet.

Toyo lists a maximum load of 1301 pounds. At what inflation pressure? Different cars have different stated maximum loads. Our Pontons do. How might this affect our comparisons?

Finally, the Toyo blurb states the tire RPM to be 842. How can that be? Note the diameter is listed at 24.7 inches. (For calculation purposes I assumed, despite the uncertainty above, that Toyo meant on the car and inflated with the car loaded to specification.) The 24.7 inches diameter equals 77.6 inches circumference (recall: C = Π d). Dividing the circumference into the number of inches per mile (63,360) yields 816 RPM. Something was wrong with the Toyo data. Besides the tire was, at least nominally, too fat for the spare wheel well. I forewent the Toyo even though I could buy them at $53.00 each.

Other comparisons

I also checked the Cooper Tire data sheet for its 185/80R13 tire. The stated section width is 7.2 inches or 183 mm. The "overall" (whatever that means) diameter was 24.5 inches. The RPM figure was stated at 846. Calculating from the diameter figure, the tire RPM would be 823. Although no more than a three-percent difference, I remained puzzled by the discrepancy. The Cooper also was not much less expensive than the Toyo.

After finding similar anomalies on some of the other tire brand data sheets, I tried Sears. Sears sells the Guardsman III tire in the 185/80R13 size. It is a low-price tire with a comparatively low tread wear warranty. I found the same sorts of problems on the data sheet available only on the web. The "overall diameter" was 24.7 inches. No RPM were stated. As with the Toyo above, the diameter figure of the Sears Guardsman calculates to 816 RPM. Physical measurement resulted in an 811 RPM figure.

Annex 2 - Many Variables

Tires

Mercedes-Benz supplied various brand tires originally. They doubtless were not identical with one another and also were not necessarily precisely to the norms stated in the Service Manual and the TDM. One production run by the same manufacturer may have been different from a subsequent one.

Presumably new tires are used when making the technical measurements. This means a tread depth of generally 10/32 inches. Double that for each side of the tire and it comes to 5/8 inch or 16 mm. As the tire wears, the RPM figure changes. Speedometer/odometer accuracy is thus affected too.

Crossply tires expand when in motion and hot, at least to some degree. Review the several dynamic radius figures in the Type 190 Service Manual. At each higher speed increment this radius increases about two percent. Where is the reference point for comparison set? As discussed above, how does this compare and contrast to the data for modern tire manufacturers which, in turn, may not be directly comparable one with another? When hot, modern radials expand very little compared to cold. How, though, does expansion due to centrifugal forces compare, radials vis-à-vis crossplies? My suspicion is that it is nothing like crossplies. Note that modern tire data tables are based on stationary (static) tire measurements, not rolling (dynamic) measurements as in the 190 Service Manual. What are the measurements for the original Ponton tires mounted but at rest, not in motion? We do not know.

Speedometers/odometers

Several different speedometers/odometers were fitted to Pontons. Cars with Hydrak transmissions[13] received a different one from their non Hydrak counterparts. On some cars different rear differential ratios were offered, and these required different speedometers/odometers. There were mile and kilometer versions. Cars originally with 15-inch tires got a still different speedometer. (I do not know about the rare 14-inch option.) These all had to be calibrated to the RPM norm of the available tires. For a full understanding of the variety of speedometers/odometers offered, the spare parts catalogues published by the German Mercedes-Benz Club (Mercedes-Benz Interessengemeinschaft eV - MBIG) provide a detailed list.

Speedometers/odometers are notoriously inaccurate and inconsistent when new. Mercedes-Benz was no exception. One report, for example, suggests that 190SL speedometers/odometers had on average five-percent deviation from true. Glenn's Mercedes-Benz Tune-up and Repair Guide cites Road and Track reports indicating significant, different deviations for several vintage Mercedes-Benz when new.

Many decades have passed since Pontons were new. Some may now have replacement speedometers/odometers. If yours was replaced, can you be certain it was the correct one per the original fitment? What if your car was converted from Hydrak to non Hydrak, as many were? Was the instrument cluster also changed at the same time?

Over time speedometers/odometers can wear and change readings vis-à-vis new. The problem is that the speedometer may not change commensurately with the odometer's change from original. Inside the instrument cluster the drive mechanism for the speedometer is different from that for the odometer. It is quite possible for an original Ponton speedometer today to register too fast at the same time the odometer registers too slow - when both are derived from the same RPM of the tires. Both can measure too fast or too slow but by different amounts. Measured mile and time readings can demonstrate this.

So, which do you want: an accurate speedometer reading or an accurate odometer reading? You may be faced with having to choose when determining which modern tire RPM is appropriate for you. All this is why rallies start with a measured section to calibrate each car.

Wheel rims

Some Pontons received 4.5-inch wheel rims originally. Others got 5.0-inch rims. The difference is 0.5 inch. Both sizes were available on all Pontons except the 220a, 220S, and the 220SE, depending on the tire. Refer to section 40-0/2 of the Type 180-220SE Service Manual for specifics.

Which rim you have can affect the aspect ratio. "It is generally accepted that for every 0.5 inch of wheel rim width change the section width will change 0.2 inches (or 5 mm)." See: http://www.mickeythompsontires.com/avon/tech_conversion.html. This suggests that a radial tire on a 4.5-inch Ponton rim will measure about 5 mm narrower than the same tire on a 5.0-inch rim. One final variable for your consideration!

Footnotes

[1] 1 in. = 25.4 mm.

[2] Π = pi = 3.1416.

[3] Doubtless there were some variations in the field. Mercedes-Benz would fit originally tires by different manufacturers. There are indications that tires fitted to Pontons delivered in the United Kingdom, for example, had greater diameter than the published norms.

[4] See page 287 (Group 40, Disc Wheels and Tire Size) of the 1960 TDM reprint.

[5] Did this mean that as early as 1960 Mercedes-Benz had declared the 6.70-13 size obsolete? How, then, was it that this size was fitted to some fintails in the 1960s? Why the apparent contradiction? Henry Magno (a restorer in Massachusetts) provided the likely answer. The original TDM was a collection of loose leaf pages in a binder. From time to time Mercedes-Benz issued updates and corrections. Apparently the page in question got such a replacement sheet that stated the 7.25-13 substitution. When the TDM was reprinted, this page, being the most recent version, became the final one.

[6] Here I have to confess that I did not search for a standard definition of what constitutes a 13-inch wheel. Is it measured across the hole in the tire? Is it measured from a specific point on the wheel rim? I punted and simply used a flat 13 inches as the figure for calculation. Anomalies here could affect the calculations.

[7] From everything I could find I conclude that the section width of the original tires was exactly as stated. The 6.40-13 tire had a section width of exactly 6.4 inches. The 6.70-13 tire had a section width of exactly 6.7 inches. The 7.25-13 tire presumably had a section width of exactly 7.25 inches, but I cannot be certain of that.

[8] That assumption, of course, is open to question. Note the comment above that no Mercedes-Benz data were available on the historical norms for this tire. I make this assumption because Mercedes-Benz did not change the speedometer/odometer to accommodate the standardization change in the tire size.

[9] The black + white photo was taken from James Taylor, Mercedes-Benz since 1945, (Vol 1), Motor Racing Publications, Ltd., London, 1991.  The photo of the red 190b sedan with people on the steps is a factory sales brochure.

[10] Using the same measuring tape, I measured a dismounted identical tire at a Sears store. It was less than 1/8 inch smaller in circumference than the inflated tire. I was surprised as I had anticipated a greater difference.)

[11] op. cit, (the work just cited by) James Taylor.

[12] The web data for the 185/80R13 tire circumference had proven very close to the measured circumference, so as with the 175/80R13 Guardsman above I assumed a similar confidence level with the 165/80R13 tire measurements.

Long after writing this article I bought a Mercedes-Benz Type W105 219 Ponton sedan that had a very old 175/80x13 tire as a spare. I measured it carefully using the same techniques and devices I used on the 180/80x13 tires on my garage floor. It was a Techna R-3000. The section width measured seven inches or 177 mm. The circumference measured 75.5 inches or 192 mm. That calculates to 839 RPM. This tire measured larger than I had expected and obviously blurs the distinction between the 185 and 175 section width size category. It also defeats my (now erroneous) conclusion that the 175 number was an absolute maximum for the category. This means that one can have even less assurance that the tire in size 187/80x13 and/or 175/80x13 one may be considering is what it is touted to me. It reinforces the point that one must make physical measurements.

[13] Hydraks were a sort of semiautomatic clutch supplied to some six-cylinder Pontons. I do not know why on Pontons they required a different speedometer/odometer from their non Hydrak counterpart, but they did.