Measured "running" tire circumference VS Manufacturer's numbers

I have a dilemma that I can't seem to get my head around. . .
In calculating gearing options for a modified vehicle, I am using the measured distance from the ground to the center of the axle of a pneumatic drive tire.
I figure that is the lever arm and is the ONLY number that should be used to determine my distance traveled.
(Radius (distance from ground to axle center) X 2) X pi)) = "running" circumference.

However, several folks keep telling me that I [U]must[/U] use the tire manufacturer's calculation for that size tire (which is not the same as the one I have measured).
While the differences are slight, they do change the final ratio, and I'd like to know if I am misunderstanding things, or am correct in my calculations.

Due to variations in manufacturing, the as-built 'pneumatic drive tire" will measure different than OEM specifications. Moreover, any loading applied to the tire may cause significant radial distortions at rest that may go away at rated velocity due to centrifugal force.

I recommend you use the manufacturer specifications. You may consider doing your calculations with the specified largest and smallest radius of the tire.

The circumference is equal to:

Pi x d

or

2 x pi x r

where pi = 3.14.157 and d = diameter and r = radius

I agree with Kelly.

No matter how much deflection the tire encounters, one rotation of the tire will travel the distance of its circumference (not including tire slippage). Think of a tracked vehicle (like a bulldozer or tank). One rotation of the track travels exactly the distance of the length of the track.

With a tire, however, the length of the "arm" as you mentioned will change due to loading and wheel speed (centrifugal force). As the length of the arm changes, so will the amount of torque being transferred to the ground. However, the distance traveled per tire rotation will be the same as the circumference of the tire.

In support of the above, the maximum rolling circumference and diameter is based upon the molded in tire belt. These belts, being either kevlar or steel (generally kevlar for road tires) are strong enough to limit the maximum tread diameter under centifugal force to the manufacturer's specification. This is the reason that super wide racing tires don't deform into a complete torus shape at high racing speeds.

While you can use the manufactures specs, keep in mind that those numbers are for new tires. As your tires wear overtime, you will end up with a smaller diameter and thus shorter circumference.

To the point of measuring, if you measure from the ground to the axle you will have inaccuracy because of the flat spot the tire forms with the ground. A better reading would be from any point around the tire that is not touching the ground. The best way, although much more hassle, would be to take the tire off the vehicle and measure.