@'s a PITA!

the dyno doesn't care what car is on it, or what tyre its got. the dyno simply measures the force seen at the rollers (torque or tractive effort), multiplies that by the roller speed, divides it by a constant (5252 for imperial bhp) and then it has the bhp figure.

no adjustment is needed to the dyno if you change tyre size. eg a 10% larger tyre would produce 10% less torque measured by the roller, but the roller will also be going 10% faster, so the mathematical equation power = torque * rpm / 5252 remains unchanged. The dyno will give the same power output no matter what tyre you put on a car.