Originally Posted by sabino56
The effective radius of the tire will be the distance from the center of the axle to the ground.
This is the effective radius when calculating acceleration, but it is not the radius you need for speedometer calculations. Consider the tire as it is often pictured in, say, a Walter Lantz cartoon. It can be terribly deformed, but, as the wheel makes a complete revolution, all that rubber has to pass under the axle. And, so it is with a real tire. The circumference you measure as the tire is mounted on the wheel at the tire shop is essentially the same circumference you experience as you drive down the road. Now, there is some expansion with speed. With the old bias ply tires, it was considerable. As I recall, it amounted to 0.2 wheel revolutions per mile per mile per hour. With radials, however, it is almost inconsequential. So, with radials, the "rollout" method (as has been previously described) is probably the best method for determining that which you need.
(In the late fifties, my 8 to 5 job was sitting in front of a mechanical Friden calculator, calculating miles per gallon and acceleration for Chrysler Corporation products. Surprisingly, I didn't find it all that boring.)