Aww the joys of buying a used kenworth

Someone has changed something. Which engine? You need for someone to do the calculations on the gear ratios and tire sizes as changed from factory specs. Once you determine the tire revolutions per mile the ecm parameter can be changed/programmed. From there a road test to verify everything is correct with GPS or mile post check. Fine tuning can be done if you run with the laptop and change on fly.

 

Do you not know how to do the math to figure out your rpm for a given speed?

 

I bought a used truck. Speedometer was off by 4mph. Mechanic put reader on truck and it had 500,000 more miles on original ECM than the odometer. I'm pretty sure my odometer/speedometer got swamped out.

 

A few things that could throw your speedometer off are:

-Tone ring on rear of transmission replaced with a different one.
-Rear end ratio changed.
-Tire size changed.
-Wrong calibration of the speedometer/software.

 

My speedometer changed when I went to a different tire size, I expected it. There is a formula to figure out, tire size, rear end gear ratio, and final drive on the transmission. Once you figure all that out, you have to find the sheet that converts that into what you program in the ECM,(it’s been awhile since I did mine). Anyways
That got it close but no cigar. I had to figure out a similar formula and adjust the “Dip” switches on back of the speedometer head itself. That finally got it on the money.
So, to answer you question, those are the 2 things you need to mess with to get your speedometer right.

 

you do understand that you truck has 12% more miles on it from what they say it was at the dealership