Another update..

For reference, the ACTUAL set up on the truck is 37/13.5/R20 with 4.88 gears.

After more trial and error test runs, here are my results:
(These are the settings in the tune via Diablo Predator, and the +/- of the GPS speed compared to vehicle speed while traveling at 50mph ...

Ok, so I finally got time to plug in the tuner and poke around.. First off, it appears this tuner is very old (menu says build date 09/02) and the software version is 1.03 and hardware v1.00. I cannot update it because I got the tuner passed along to me with the truck used and all i have is the ...

Alrighty will do won't have time until tomorrow but I'll report back then.

I found an older thread on here where someone had an issue when swapping gears and tires too and ended up having to have a custom tune written for it. I suppose if retrying doesn't work I may be headed in that direction?

New to the foum here, and just picked up a 2002 GMC. It's the half ton 5.3L. The speedometer in the truck reads around 10mph too fast at highway speeds. (displayed speed 70mph, actual 60mph) The truck does have some modifications.. It has about 10" of lift, and sits on 37" tires on 20" wheels with 4 ...