I have a 96 Vortec 5.7 and the speedometer runs about 15 mph over what the truck is acctually doing. The mile counter seems to be correct at counting the miles. I hear that there are 2 sensors one for the odometer and one for the speedometer, is this true? Also any suggestions of what could be wrong or a quick fix, any helps appreciated, thanks!

If the odometer is correct, then chances are you have a problem with the speedomoeter itself. They both use the same signal from the PCM to determine the speed and distance traveled. Is it off 15MPH across the entire range? If so the needle may need to be re-oriented on its shaft.

it slowly climbs up past 30 miles per hour 30-55 it ends up on 70 when you are driving 55