Measure your rear tire from the ground to the top of the tire, you will need to know the transmission and engine also......
go here:
http://6066gmcclub.com/forum/showthread.php?t=48090
posts #6 or #7
you can see how your truck performs now as it is equipped and if you changed out the transmission, engine, rear axle ratio etc.......diameter of the rear tire in inches is in the file name so if it is 24" click on the file name that has 24 in it.
speed is also shown at 1500, 2000, 3000, 4000 rpm......