I have an 02 Excursion Limited Diesel and cannot get the speedometer calibrated. I purchased the vehicle with 35" tires (original equipment is 32") The tires were about 75% worn and the speedometer was off by a consistent 5% at any speed (reading fast). Recently I switched to 33" tires. Now of course the speedometer is now off by about 15%. I have a scanner (AutoEnginuity) which allows me to go into the ABS system and change the tire size. I can hear it setting and my settings are saved from previous session. But no matter what I set it to, the speedometer doesn't seem to change. I tried going all the way down to 31" tire size and it maybe changes so it's reading about 80mph at 70mph GPS (2mph difference). Or, maybe it was my imagination.

I spoke with the stealership, and they basically said they have no idea what would cause the PCM to not affect the gauge. They perform the same process when they reset it. It also may be worth noting, I have Hydra 16 position tuner installed with custom tuning. However, I don't believe that tuner touches tire size. I can confirm that with my tuning tech. But, I believe he just uses the size which is inputted into the PCM and adjusts shift points accordingly. Or, maybe I should attempt to do this with the chip unplugged regardless? The chip is the only thing I can think of that could interfere aside from some sort of inline correcting module like a TrueSpeed device. I took the dash off looking for said device. I checked around the battery on the driver’s side, around the firewall and around the wheel well for an aftermarket device. Couldn’t find anything. Is there anywhere else I should check? Any other reason I cannot seem to calibrate the speedometer like normal?

  • Are you still experiencing this issue? Jan 4, 2016 at 21:13
  • Yes. So, I was able to get it back to about 8mph off at highway speeds. I fully inflated my new tires (Less Schwab had my new class F tires at only 40psi, so I put them to 65psi). Also, I've replaced the ABS sensors on the front as part of my front end rebuild recently. Still no difference. Adjusting the PCM is effecting it a little... if I turn the tire size all the way up, the speed is way off! Like 15 mph at 70mph. Right now it is all the way down (31") which makes it most accurate but still WAY off.
    – maplemale
    Jan 5, 2016 at 17:22


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