Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
GM says that due to the nature of these speedometers there is a degree of variation that cannot be corrected. They say that 4 MPH off at 20 mph, 3 mph at 45 mph and 2 mph at 65 mph is acceptable. Would you say that yours is off more than that?
Original tire size right? Does it say you are going faster or slower than you really are?
Have you had it in to the dealer to see if there is a re-program that can be done? There is nothing listed on the GM database saying that they have identified a problem, but they could have something that they have not yet published.
Are you the original owner? You aren't using any kind of chip or programmers right?
Do you have a scan tool that will monitor the vehicle speed sensor?
A scan tool is used to run diagnosis on the vehicle. If there is a problem with the speedometer the scan tool will be the only way to tell for sure. It can pull up the signal from the vehicle's speed sensor and see if it matches with the speedometer. If the speed sensor is correct but the speedometer is off then you know the problem is in the speedometer.
Perhaps you could list all of the things you have tried so that I don't suggest them to you.
It's hard to know where you are in the diagnosis process without you telling me and it threw me off a little when you seem unfamiliar with terms such as "scan tool".
I can opt out and see if someone else is willing to help if you would like.
Sorry, they dropped your question to my timed out list for some reason and I didn't see your responses.
You should be able to have the dealership reprogram the PCM to match the gear ratio in the differential. They will need to know the ratio- do you have the info on what gearset was put in there?