> Too inaccurate. Do/run a comparison of 5 tires on each calculator and see
> the difference.
>
>
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