

I have test all three methods. GPS is the best, but it has drop outs. You can add an inertial gyro system to compensate, but that becomes sloppy the longer it goes without GPS.
The tire method has a lot of variances, but the measure at the transmission is often worse.
Well yeah. My bicycle odemeter has settings for different size wheels.
We used to take vehicles in for calibration and then all runs had to use the same psi in the tires.