Just as a piece of triva,
...and for those who are really into tires...  a 265/70/R15 tire has a width of 265mm (10.43 in) and a height 70% of its width which is 185.5mm (7.3 in).  That gives and outside diameter of the tire on a 15 inch rim to be 29.6 inches.  The 31x10.5R15 tire diameter is 31 inches (~30.6 depending on tire) and has a width of 10.5 inches (+/-).  That translates into a 3% increase distance traveled per rotation with the truck tire and a corresponding  3% increase above indicated speed (or error).  Also, the odometer will show 3% less than the actual miles traveled.
Regards,
Malta