I have a '97 F150 XLT that the previous owner fitted with the equivelent of
a 32 x 11.50. The tire size is not the point, the problem is my speedo is
out of whack by a significant margin. A cursory test using my watch and the
mile markers on the side of the freeway says that 70 on the speedo is closer
to 80 on the ground. (A mile takes about 45 seconds at 70, it should take 52
seconds.)

I'm aware of a gear in the tail of the transmission that one changes to
offset the affects of oversize tires, but it seems my F150 gets the speed
information from the diff. If this is true, how do I correct the speedo
error without paying the King's Ransom that is demanded by my local
stealership?

4WheelParts apparently sends its customers out with speedo error because the
guy at the counter did not seem interested in taking my money to fix the
problem, and he was not aware of whether the computer has to be re-educated
or if a gear is changed. The dealership service writer also scratched his
head at this one, but said that if the computer training was the answer, he
would charge $90 for the service.

How is this issue addressed on my F150, does a gear get changed out, or is
it necessary to train the computer for the oversize tires?