Originally Posted by stevehnm
Why they can't align the speedometer with the odometer is a puzzle. Odometers are generally about 2% slow with new tires, so with 1.4" tread and 25" diameter they are right on when the tires are bald. The speedometer is right on with new tires and then would read 2% fast with bald tires. (I don't know anyone who drives their tires until bald, but maybe that's just me...) Maybe the speedo/odo differential is a safety feature (?)
Basically. As I understand it, there's a legally allowable range of speedometer error... I don't know the actual numbers, but let's say -5%/+10%. So you make a speedometer that is usually accurate ±7%... but this falls outside of the allowable range. So you adjust it so the speedometer reads a little fast so even the ones that fall at the ends of the accuracy range are within the legal tolerances.