Re-Gear Calculation

Hey TW, supposedly my truck was re-geared by the previous owner. It's on 35's and does 2100 rpm at 60mph. Can anyone point me towards a gearing calculator or explain how to do the calculations?

 

You didn't give the specific tire brand/model, so let's assume a common 35x12.5-17" BFG AT KO2, which per Tire Rack does 602 revs per mile. 60 miles per hour is 1 mile per minute, so the tire is spinning at 602 rpm while the engine does 2100 rpm.

You're in 5th gear with the 5-speed auto? The 5th gear is 0.72:1. So 2100 / 0.72 / 602 = 4.84. Your axle ratio is probably 4.88... if 60 mph is by GPS and not by speedometer.

 

One of these graphs might help

https://www.bing.com/search?q=gear+...f4cc04eedb8c2bdae7aa1acaa&cc=US&setlang=en-US

 

Or old school. Tape a piece string to the driveshaft so it’s hanging straight down. Jack up the rear wheels off the ground and spin one tire a complete revolution. Count how many wraps of string around the driveshaft. Close to 5(488) about 4 1/2(456) barely over 4(410)

 

http://www.grimmjeeper.com/gears.html

Also, look up the specs on your tire. It's probably closer to 34 than 35.

 

Since it's an open diff, I think you need to either chock one of the wheels or have the locker engaged.