Just picked up an 89 f350 dually. 7.3 5 speed. What is easiest fix to get higher speed at lower rpm? Change rear gears? It has a Dana 80. Just seams to be screaming at 55 mph. Tach does not work

gears make a big difference. whats in their currently?

I haven’t been able to figure that out yet no tag on axle.

Door tag or internet search would tell

Jack the rear end up so you can spin the rear wheels. Tranny in neutral. Mark the driveshaft with a chalk mark. Spin the tire one revolution and count the number of times the driveshaft rotates. Driveshaft rotates 3 1/2 times you have 3.50 gears. 4 times and you have 4.00 ratio. Etc.

Tom

So I jacked up rear end and drive shaft spun 2.5 turns. Does that mean it as a 2.5 gears? Never heard of that low. And doesn’t help me change gears to drop rpm?!?

It wouldn't be 2.50. My guess is it turned 3.5 times. Make sure you rotate the wheel one full rotation. I normally use the valve stem and start with it down at 6 o'clock. A chalk mark on the driveshaft makes it easier to see.

Tom

So it has 5:13 gears in it, what are my options for lower gears? Any good aftermarket stores?