Pontiac Repair: speedometer, pontiac grandam gt, size tires

Question
I have a 2004 Pontiac GrandAm GT and the speedometer is 20 mph over the actual speed. It reads 90 mph when I am actually going 70 mph. I know it's all electronic and if it's tampered with you can tell, so do you know what I can do to fix it? or have any suggestions?

Answer
Hi Tammy:

well first off does this car have the original size tires on it? if not that will thropw off the readings but I doubt 20 MPH. you need to have this car driven with a scan tool on it and see what the computer sees as vehicle speed VS what the speedo reads. also the ABS wheel spped sesnors can be used to compair the speed to the speedo and see where the issue is. It may just me the speedo is off but if the engine computer sees the speed greater than it is then you have to start fishing around for a possable cause like maybe this doesn't have the original transmission it it??? good luck :)