For years we fought this but now it's so easy it's almost a non-factor. GPS-GPS-GPS. It's dead accurate no matter the tire size or gear ratio. Right now you have too many teeth on the drive gear and/or too few on the driven. Stick the GPS in the truck and see what PERCENT the indicated speed is off from the real speed on the GPS. Google "speedometer drive and driven gears" and find what drive/driven combination will reduce the driven gear by that percent. The early small gears went by tooth count and the later big gears go by count and/or gear color.

Evan