Overdrive Transmission vs. Re-gearing

[Jeepsy Soul 71]

Hi all,

I've got a question that hopefully someone has some experience with. I recently bought a 1981 Jeep CJ7 (AMC 304 V8) with a 4 speed manual Transmission . I noticed that at 40Mph, it's running 3000 RPM in 4th gear. Seems like it needs another gear/Overdrive to bring the RPMs down to a sane level. I'm told that I can re-gear the front and rear differentials to bring the RPMs down at highway speeds, but was wondering if it would be easier/cheaper to find a 5 speed manual Transmission to accomplish the same thing. Note: I don't have a clue what the current gear ratio is. I'm told you have to remove the rear diff cover and count the ring and pinion gear teeth to figure that out for sure. I'm also running 33" tires and don't want to go any taller or I wont' be able to climb in! So, any thoughts on the matter would be appreciated.

 

[mtnwhlr]

I would really like to know what gears you are running now that is giving you 3000 rpm at 40mph in 4th gear. That doesn't right to me.

 

[Posi]

3000 rpm at 40 MPH with a 4 speed isn't possible in a stock CJ
Is the Transfer Case in 2 high?
Need more info, do you have stock axles? Has the speedo been recalculated for 33" tires?

 

[caveman]

The lowest gears that you can get over the counter for a stock CJ is 4.88 to 1. With this ratio and with 33 inch tires at 3000 rpm you would be traveling at approximately 60 miles per hour. I'm thinking that Posi might have come up with your answer.

Sent from my LG-LS777 using Tapatalk

 

[Lumpy Grits]

Take it out of 'low' range.
OLG

 

[mriplaybass]

I ran the numbers, and you would have to have 7.35 gears to get that kind of rpm at 40mph assuming a 1:1 output on your Transmission and Transfer Case . I believe that the Dana 300 has a 2.62 low ratio. If you are running the kind of numbers that you stated, a 3.07 axle ratio would give you around 37mph at 3000rpm if you are running in low range.

 

[cglax6]

Hi all,

I've got a question that hopefully someone has some experience with. I recently bought a 1981 Jeep CJ7 (AMC 304 V8) with a 4 speed manual Transmission . I noticed that at 40Mph, it's running 3000 RPM in 4th gear. I'm also running 33" tires and don't want to go any taller or I wont' be able to climb in!

Given those numbers, you're diff gear ratio would have to be around 7.3:1 for a 1:1 4th gear Transmission .

It sounds like you're in Low range on your Transfer Case . Shift to High range.

If you have a 1:1 4th gear Transmission , a Dana 300 (2.62:1 Low) in your CJ, and if you're getting those kinds of numbers, and if you're in low gear, then you most likely have 2.73:1 diff gears.

2.73 x 2.62 = 7.15

Close enough to the 7.3:1 ratio that you'd need to get 40 MPH @ 3000rpm with 33" tires. 7.15:1 would put you at 41mph with your numbers.

If that's your problem, then you may want to move to numerically higher diff gears if you want to take it to the trails. 2.73 is great for highway driving, but that's about it. And that's really pushing it with 33" tires...especially if you have 2-piece axles.

Let us know what you find out.