Sorry John, the manufacturers know the revs/mile from actual testing. No
math trumps real world counting. Michelin gives revs/mile at 45 mph.
Various errors can come into play because tires of the "same" nominal
size actually vary quite bit but have to be assigned whatever nominal
size they are closest to.

The size 195/75R14 is closer to stock than the 185/75R14. Stock size
depends, as 185R14C and 205/70R14 were both stock sizes in North
America. 195/75R14 fits between them in revs/mile.

Mark

John Bange wrote:

> Not necessarily inaccurate. All the others might be wrong! Unless there's
> some unknown adjustment in the math they use to calculate a more accurate
> "rolls per mile" to account for some variance in something I've never heard
> of, the miata.net calculator is dead on mathematically accurate.
>
> 205/65R15 = 205 width, rim-to-tread distance is 65% of 205mm, or
> 133.25mm (times .03937 = 5.24606 inches).
> Radius of tire&  rim = 7.5 inches + 5.24606 inches = 12.74606 inches
> Circumference = 3.1415927 * 2 * 12.74606 =80.08588 inches
> 1 mile = 5280 feet = 63360 inches
> 63360 inches / 80.08588 inches =  791.1507 revolutions per mile.
>
> FWIW, i think the 185/75R14 is the modern equivalent of the factory tire,
> and it calculates to 809.
>
> The TireRack.com spec numbers (which come from the manufacturer cut sheets
> like everyone else's do) don't even match each other. They have the diameter
> a half inch LARGER than the math would have it, then quote a RevsPerMile for
> a tire that's SMALLER than the math comes up with. I admit there's the
> possibility that the manufacturer is accounting for some sort of rolling
> radius deduction due to deformation, but my real-life experiences have
> always come up closer to the mathematical RPM than the quoted one... but of
> course the plural of "anecdote" is not "data". In a very real sense Your
> Mileage May Vary.
>
> --
> John Bange
>