Originally Posted by

**Bill Kapaun**
And they all pretty much use circumference DIRECTLY!

I don't know why you want to go off on a tangent? Did you just learn about PI?

2% is 2% no matter what method you use!

It's the radius (

__the height of the hub center from the ground__) that is affected by the load on the tire (and the air pressure).

More-weight/less-pressure means a smaller radius (the hub center is closer to the ground).

It's the

*radius*, at that

__one__ place, that causes the difference in the

__apparent__ circumference.

The radius is key to understanding why measuring the circumference of the tire isn't really a good approach and why the roll-out measurement is.