Question 8 1/4 rear end

Another method for lazy people like me is ;
If you have a tach, an accurate speedO,and a manual trans, it's real easy. Just run it up to 50mph in top gear and record the rpm. Then a little math will spit it out.
With an auto you might have TC slip to deal with, and that will throw the math off. So it's best to finish at an rpm higher than the stall speed. So then you would probably use Second gear. The trick is now you have to know your second gear ratio.
In both cases accuracy will be improved by knowing the tire roll-out, or circumference.