Here is a formula for figuring speed if you know your gear ratio, tire diameter and RPM.
MPH = RPM X tire diameter divided by: gear ratioX336
If you have a 40" tall tire and your engine is turning 2500 rpms, with a 7.17 gear ratio, the speed would be 41.5 mph. Of course this assumes that you're in high gear and it's a 1:1 ratio, I doubt that you have overdrive in that truck.
I used to have the formula for figuring big trucks, using the tire revolutions per mile but I can't find it; if I run across it I'll post it. It was important to me when I had trucks on the road, now it's just trivia.

'65 C-10, 65 L-60, 65 C-80, 72 C-60 & and a bunch of old Chevy cars.
Never time to do it right, always time to do it over.