Very interesting Steve. I believe there is some sort of law that requires a speedometer NEVER show a speed lower than actual speed. Also... As the tire wears the diameter decreases thus resulting in a lower speed than when the tire is new. Putting on miles "faster" also means sooner (and more) service intervals for the dealers.

I checked mine with GPS and it is off by about 5% with a tire with 11,000 miles on it. This can be corrected via the ECM but when? When the tire is new? When the tire is old? In the middle? What about a different make and model of tire? It's all a compromise...

 

I have always set my personal bikes to read correctly when the tire is half worn. That gives me the least error over the life of the tire, typically within 2 mph each way. Why HD does it the way they do, who knows for sure but they typically read 3 - 5 mph too high. This puts more miles on the odometer as well as the wrong speed displayed. The '24's are no different from what we have tested in house.