i have 37 inch super swamper tsls, and 65 on the highway in 5th gear is about 3000 rpm. the speedo seems to be about spot on whenever i pass police scanners.

 

Jack it up block it up put the t case in neutral mark the drive shaft and mark the tire/ fender (peice of tape) turn the tire one complete turn counting the number of times the drive shaft spins
4 complete spins is 4:10
5 and 3/4 spins is 5:71
And so on
I might be wrong tho

 

Btw do it on the front and the back to make sure they match

 

Thanks! Found out they are 5.29s thanks!