Speedometer gear?
 

Just put slightly larger tires on my '98 F-150 4x4 with E40D tranny (went from stock P255/70R16's to LT265/75R16's).

Even with the stock tires, the speedo was off by ~3 MPH (speedo read slow). Now with the larger diameter tires, the speedo is probably off to the tune of ~5 MPH at highway speeds.

Called the dealer to see if they could reprogram the PCM to compensate for the difference and they said "no". I then called a shop that specializes in speedo calibration and he told me to first run a measured mile on the highway (between mile markers) to determine how far off the speedo really is. He then said that I could change out the speedometer gear in the tranny, using a gear with more teeth, to compensate.

Has anyone done this with an E40D tranny? Where is the gear located, and how difficult is the job? What part should I request when ordering from Ford? Thanks!

This is the site that will help you the most, the table will actually give you the most likely gear.
http://www.finer.com/internet/pages/speedo.htm
You have to change the gear at the transmission on your year.
I would get the two closest gears just in case.
I never found the "measured mile" to be of much help.
I had a friend that had a new truck drive down a level road using cruse control. We used those long range walky-talkys that are out now to communicate and we checked speeds at 5 miles intervals (30-35-40-45 ect). I got mine pritty close that way.
Good luck.