I found a good truck, A 94 Ford F-700 12V BT 5.9 Cummins(190HP) , 6 speed trans
The Truck now has a 6sp (direct 6th gear) 5.29 gears and 11R22.5 tires.
And the tires I have now have dry rottted side walls and MUST be replaced.
THe box I just mounted is 1 inch above the frame so It looks like I must go to lo pro 22.5 tires. The brake drum on the air brakes are way too large to let me go to a smaller wheel. This thing is spinning right at 2800 at 65mph or so right now.
What I have now: 65mph X 5.29 X 336/ 41.3 = 2797 RPM
Tell me if I'm going too far:
36.5 tire + 3.90 gear @65MPH = 2333 RPM ?
Or should I go to a 3.70 gear? 65= 2247 RPM ?
The formula I'm using is
MPH X GEAR X 336 / overall tire height = RPM