Could Someone tell me why the odometer and ECM numbers don't match

The speedometer head has probably been replaced sometime in the past, either 587,000 miles ago with a new one, or sometime later with a used one, or the ECM could have been replaced with a high mile used one, but I would bet on the speedometer being replaced, and the ECM being correct..

 

ck serial # on ecm/motor/frame to see what has been changed. # of ser.ECM is also on side of motor.

 

both answers are correct, usually what happens, is the cluster goes bad, and is replaced with a referbished or new cluster pack, Usually when a used ECM is used, its reprogrammed for your unit, ( did that with my detroit 60 Freightliner) so ECM read 0 miles

 

Thanks for making this post. Not trying to steal your thread but why can't speedometers be reset as well as ECM's? Light trucks & cars can be reset legally by filing paperwork with the DMV. There is always an in Craigslist for having this service performed on cars & light trucks. I called the guy doing it and he said he don't have the equipment to do big trucks. All the trucks I've ever owned have had different ECM & speedometer mileage readings.

 

I've never seen any truck where ECM miles perfectly matched odometer miles. Going by the GPS my odometer, before it quit working this past April, was spot on 100 miles driven when the GPS said I had 100 miles to go. For whatever reason the ECM would go 102 miles for every 100 GPS or odometer miles. When they were both working and at 1.1 million miles on the odometer there was about a 5k miles spread between odo and ECM. It is what it is. Have often wondered if worn down tires don't throw the ECM off? A tire with 32/32's compared to one with 16/32's is a half inch overall diameter difference. Whether or how that would make a difference, I don't know. Now an ECM and odo with 500k spread, one or the other has obviously been changed out.

 

That's exactly why there are devices like the speedohealer. I have one on my R1 because the front sprocket is not the stock tooth count and the rear tire is a different size than stock. That threw the speedo off like 18%. On my car I put bigger wheels/tires on and the speedo is off maybe 5%. Not sure what worn tires would do to the reading on a semi but even a small amount adds up over time.

How do the current speedos get a reading? I whould have thought it came from the ECM so if one was off they would both be. My '89 doesn't have all that fancy stuff.

 

Or the speed sensor on the back of tranny stops working. Mine stoped in atlanta drove to st. Louis , my cruise, speedometar and engine brake didnt work , and id didnt register none of those 600 miles.