I have a 1995 GMC Sierra Z71 4-wheel drive with a 350 V8. It has over 300,000 miles. Discovered that when the speedometer reads 60, the truck is doing only 50 mph. Changed the Vehicle Speed Sensor with no results. Most of the engines sensors where replaced 2-3 years ago for various reasons. Truck runs great. The speedometer does not flutter. I called to a calibration garage. The Mechanic said it may not be fixable and could cost nearly $500 besides. I hope there is something that can causes this problem and can be fixed. I just need someone with knowledge in this area to give me a path to try. Thanks for this forum.

 

The difference in speedo reading could be from a couple of things. Tire size or a different speedo drive gear in the trans could impact your readings. Does the truck have monster tires? Tranny ever been changed? On my 88 I think there was something on the back side of the instrument cluster that dealt with the speedo - like a setting or calibration.

 

The transmission was replaced at 101,000 miles. The tires are the size that came on it from new. The speedometer has been correct until after I changed most of the sensors. When I have gone by the "You are doing this speed" units on the sides of the road the speed was correct. Have not seen one in long time. The speedometer is not 10 mph off when doing 35 mph but is about 6 mph over reading 41 mph.