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Nitpick all you like... I'm an engineer and do it for a living!
The 0.82 aspect ratio you found is a reference number. I got my .78-.80 figures by back-calculating from Michelin and Continental tire data, back when they still listed 185R-14. Not disputing your number, just letting you know I didn't pull mine out of the air. In my notes on my tire spreadsheet, I show "listed OD" as 25.59 inches for the OEM Michelin 185R-14 that my 245s came with. That works out to an aspect ration of about 0.795.
One of the risks of calculating tire diameters and such is that the calcs rarely take into account the actual loaded rolling radius of the mounted tire. Clearly that dimension is smaller than the nominal, calculated diameters/circimferences we whip out with our tire calculator spreadsheets, and in fact will vary somewhat with actual vehicle loading. An even smaller effect is that the OD will vary with the rim width - e.g., a given tire will be ever-so-slightly taller on a 5-inch wide wheel than a 7-inch wide wheel. Not quite Angstrom units, but - probably a few hundredths of an inch.
My "2 percent" error of course gives a 1.2 mph error at 60... I just rounded upward and my disclaimer was that I said "about."
The Bridgestone tires I'm running right now catalog an OD of 24.68", which is 3.7% smaller than the OEM tire OD of 25.59". In the local speed enforcement zones that show your speed (and who knows what the error those things have), they usually show 58 mph as I drive at an indicated 60. That's a 3.4% error... but, speedo tolerances are typically greater than 1 mph. So if anything, I probably should have said "about 3%" error...
But I'm glad to see someone ELSE out there looks at these things!!!
Smitty
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Thank goodness we don't get all the government we pay for. -- Will Rogers
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