Quote Originally Posted by Baxman View Post
Beware of the handy online tire calculators. They are acurate for passenger tires but not necessarily for off-road tires which are taller than same-size passenger tires.
Good point - the tire size specs for off-road tires work differently, but they would not even work with the tire size calculator I linked to.

That said, don't forget that any particular speedometer may be off to some degree - even two different cars of the exact same model & year. Baxman's speedo may have been off before he switched tires, and the new tires only exacerbated the problem.

Garmin GPS says that my VX's speedo is reading exactly correctly at highway speeds. All of our other vehicles are off by up to 4 mph at 80 mph.