i have efi live in my rig and i am running 285/75r16 on stock rims i have adjusted the speedo with that and while i am logging i am getting proper reading on my laptop but not on the speedo. at 60 MPH the dash speedo will be reading like 45 to 50 MPH. i know i am going 60 MPH because i have verified with gps. what is causing the dash speedo to go out of whack. i can pull over and shut the truck down and restart and it will work until i get to the next stop sign. then once i stop again the speedo will be off. please help i can tell how fast i am going really only because i am so use to driving my truck i can tell by the gear i am in and the rpm. i also have a 5 speed allison transmission.

The "stepper" motor in the cluster is defective. It is so common that GM issued a "recall/ satisfaction" bulletin covering the cluster assy up to 7yr/ 70k miles on certain years. A search of "speedometer" or "stepper" will yield more info.

Or, you can change it yourself. http://www.dieselplace.com/forum/showthread.php?t=140580