At 04:32 PM 3/16/2010, John Bange wrote...
>I would love to trust the manufacturer's quoted numbers, but when the
>Revs
>per Mils and the diameter they state don't match,

Where do they state the diameter (other than on the same spec sheet
where they state the revs/mile)? As has already been explained, the
diameter has only a passing relationship with the rolling radius -
tires get compressed when loaded.

When diameter is calculated mathematically, you're working from a
standardized tire section width (185-195-205, etc, so that's rounded to
about the nearest ~5%), profile (60-65-70, so rounded to nearest ~8%),
and rim diameter.

>  it's hard to take them completely seriously. I suspect they might
> use the largest possible diameter, and the smallest possible rolling
> radius.

The diameter is the inflated diameter of a tire, without any loading.
This is important to know for things like spare tire storage, wheel
well clearance, etc.

What possible reason would they have for deliberately skewing the
rev/mile figure? They measure it, under what they consider to be
nominal conditions for the particular tire. Sidewall stiffness,
inflation pressure, loading, and speed all have an effect on how much
this differs from what a calculation based on diameter would produce.
It's provided because it affects speedo/odo readings.

They're two different measurements, used for two different purposes.

>  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,

That only gets you the static rolling radius. Centrifugal force makes
the dynamic rolling radius larger than that (but still less than what a
purely mathematical calculation predicts).

>and GPS mileage vs miles clocked that assume 805 RPM).

That could work, if you get to try all the tires you're considering
buying, and assuming you have a lengthy straight road to work with.