What gears to run??

[signgrafix]

I am going to rebuild my 78 CJ5. Right now I have it down to the frame. I threw the rusted body away. I plan on replacing the 232 straight 6 with a 401. I am keeping the AMC 20 and the Dana 30 front and rear differentials. I will make modifications to them though such as 1 piece axles, air lockers etc. I plan on going with 35 to 36 inch tires with a 4 inch lift kit.

What gears should I run? 4:10? 4:56? or something else?

The uses of this jeep with be... Just driving it when I want as a third vehicle and to take on some jeep jambories. I will not take it on any trails that will beat and bang the body up or roll it over. I am to old for that stuff.

 

[Keith__C]

A quick and easy rule of thumb to find out what gears to run is to multiply the tire diameter by .13 for instance 37" tire gives you 4.81 gears. Of course they don't make a 4.81 but they make a 4.88's. I've always heard you run the next step deeper for power and the next step higher for economy, however I do NOT agree with the run a higher gear for economy statement, For real economy you need to run a gear that is going to put the engines RPMS at its sweet spot for cruising. When I changed the gears in my F-150 from 3.50's to 4.10's I went from 9 MPG to 16 MPG. The added RPMS put the motor back into the sweet spot of the power band where the motor didn't have to work nearly as hard. Of course the same thing could have been accomplished by driving faster, but 83 MPH will get a ticket 74 won't

 

[Keith__C]

Sorry, I goofed up to a 33" tire you are supposed to multiply by .12 over 33 is supposed to be a .13

 

[BullHead]

i ran my 1980 gmc with 4.10s and 38s, had good power considering the truck weighed 7000lbs, motor was 383 with 450hp

with a 401 and 36s, you could easyly run 4.10s and still get mileage on the hwy, since the v8 has alot of power, i'd change out to dana 44s at least, imo

 

[signgrafix]

i ran my 1980 gmc with 4.10s and 38s, had good power considering the truck weighed 7000lbs, motor was 383 with 450hp

with a 401 and 36s, you could easyly run 4.10s and still get mileage on the hwy, since the v8 has alot of power, i'd change out to dana 44s at least, imo

I had originally planed to do just that but, after doing some checking around, Going to solid axles and re-enforcing the housing/tubes will do just fine. Actually if you will look the dana 20 gears are bigger/beefier than the dana 44. Also, most all those hot little AMC car in the 60s and 70's came out with a AMC 20 rear end.