>
>  A tire with a section width of 210 may be called a 215, because 210 isn't
> a standard size.
>
> The problem with that calculator, and others I've seen, is that it figures
> diameter mathematically, assuming perfect circles, etc. A real world tire
> has a rolling radius which is smaller than that of a tire represented as a
> perfect circle (it compresses when loaded), which means it will turn faster.
>

I would love to trust the manufacturer's quoted numbers, but when the Revs
per Mils and the diameter they state don't match, it's hard to take them
completely seriously. I suspect they might use the largest possible
diameter, and the smallest possible rolling radius. Given that, I tend to
use the mathematical number initially, and then actual physical measurement
after they're on the vehicle (axle center to ground, and GPS mileage vs
miles clocked that assume 805 RPM).

--
John Bange