I've tried to make this coherant and well thought out...let's see how I did:
1994 Ranger XLT Super 2WD 4.0 V6. Lifted 2 inches. A4LD Automatic Transmission. Babied all it's life and pretty much good/better than new @ 186,000 miles. Came from the factory with 225/70-R14 tires. Now has P265/75-15's. Using a GPS unit and timing between mile markers verifies a 17.5% difference in indicated vs real speed ie; indicated 40MPH = real speed of 47MPH, etc.My '94 uses PSOM (Programmable Speedometer/Odometer Module). This gets its info from the RABS speed sensor on the rear differential housing and sends that info on to the Powertrain Control Module and Speed Control Module. Two units that must have correct information.
My questions involve the Programmable portion of the PSOM. How do I do that? I have a program that takes the time between mile markers and the indicated speed and gives the % of difference then explains how to recalibrate the PSOM. I quote, "Hold the RESET button while turning the key on. Then release the RESET button. Press the RESET button again and release to see the calibration constant followed by the word CAL, ie 918CAL. Enter this calibration constant, ie 918."
What RESET button? Enter the calibration constant how? Is this done by using a piece of equipment I can purchase? Is it something that's already on the vehicle (I doubt that)? Does a Ford dealer/Speedometer shop have to do this? When done, this change should get the PCM and SCM info corrected but will this also get the speedometer reading accurate? From what I read in the Factory service manual it should do that as well.
FWIW, I performed all the on-board diagnostic procedures and it was clean, no recorded DTC's.
Thanks, Jay