Dent wrote:

Back from the dead. The revolutions per mile is either a geometric calculation assuming ideal diameter, no slip between the road, and a straight line, or that calculation with a fudge factor for slip and turns, or some empirical measurement. If you assume ideal geometry, and you take the reciprocal of the rev per mile, multiply by (5280*12/pi) = 20168, then you get the diameter of the tire in inches. Similarly if you divide 20168 by the diameter of the tire in inches, you get the revolutions per mile. A 32" tire, should have 630 rev per mile. Tire rack lists the Goodyear Duratrac 235-85-16 as 31.7" diameter and 656 rev per mile. Based on ideal geometry and straight line rolling, it should be 636 rev per mile, so there is something else in the calculation. Maybe something to do with the contact patch or that the tire is not a circle. Another example is the 32 x 11.50 x 15 Goodyear MTR. Tire rack lists the diameter at 31.8 and rev per mile as 654. Based on 20168/31.8 the rev/mile should be 634. Both examples deviate by 20 rev/mile.

What you are encountering, and what they are likely figuring for is the difference between the mounted unloaded (off the ground, fully round) circumference, and the circumference calculated by what's known as "Effective Rolling Radius".

Unless you have steel tires like a train, there is always some deflection that results in what you refer to as "slip"

They figure the rev/mile from the tire mounted on a proper spec wheel, and inflated correctly, with a load properly within it's capacity.

You start with the mounted unloaded tire circumference.

Them take the loaded tire flat deflection (say, a 10" long patch) and you subtract that 10" from the length of the same 10" drawn point to point through the arc of the unloaded radius (will be slightly longer) so you have the difference.

Then subtract that difference from the total mounted unloaded circumference.

There you have the Effective Rolling Circumference.

That's what you use to figure the revolutions / mile....

As smaller tires tend to operate closer to the limits of their weight ratings, because of the nature of arced lines vs straight lines, other factors being equal, the smaller the tire diameter, the more % difference there will be between the mounted unloaded circumference revolutions/mile, and the actual Effective Rolling Circumference rev/mile