Originally Posted by RChung

Whitt & Wilson give a couple of formulas to find the equivalent drag on drums of different diameters. Both bicyclerollingresistance.com and Tom Anhalt's tests make this adjustment to convert from drums to "flat" roads. In addition, below the impedance breakpoint, the correlation of real road tests and (adjusted for drum diameter using the Whitt & Wilson equation) drum tests are quite high. For two tires A and B, if Crr(A) > Crr(B) on drums then *almost always* Crr(A) > Crr(B) on the road (below the impedance breakpoint). The only times I've seen the ordering switch is when the tires were very close to begin with. That's why as long as you stay below the impedance breakpoint, we still do roller testing. On road field testing is much more time-consuming.

That said, we likely would not have observed the impedance breakpoint without careful field testing, so it's good to be able to do them (even thought they're time-consuming and kind of a pain in the butt).