Diff 1983 cj7 258, t5, axle ratio question

[bert]

Is there a way to tell the axle ratio for a 1983 CJ7 ? I am looking to purchase one and the owner doesn't know and doesn't have access to the axle stamp. It has a AMC 258 i6 / 4.2l and T-5 Transmission and I think it most likely is all original. thanks

 

[Petescj]

Two gear ratios were available stock. 2.73 and 3.31. Only real way to know is take the cover off and count teeth

 

[CJ]

Is there a way to tell the axle ratio for a 1983 CJ7 ? I am looking to purchase one and the owner doesn't know and doesn't have access to the axle stamp. It has a AMC 258 i6 / 4.2l and T-5 Transmission and I think it most likely is all original. thanks

Since the axle is not in the jeep #2 and #4 in the document linked below are what you would use.

How to tell what gear ratio is in your CJ.

There are several ways to determine your gear ratio. Look for a tag on the differential. Pull the differential cover and look for the # of teeth stamped on the ring and pinion. Count the number of teeth on the ring and pinion. Lift the wheels...

jeep-cj.com

 

[hole]

Probably 2.73 being a AMC 258 i6 / 4.2l engine

 

[flyn hawaiian]

You can jack up da rear. get da tires off da ground. In neutral, turn da driveshaft 1 complete turn. And count how many times da tire turns. Not very accurate, but it will get you close. My 85 CJ7 AMC 258 i6 / 4.2l , had 3:31-1 ring and pinnion. I ran 12.50x33 tires. It was ok. 3:73-1 would have been perfect for street and offroading. But nothing too crazy. Good luck

 

[N9VIW]

Flyn, we always did it the opposite way. We would jack up one side, and rotate the tire one full Revolution while the other guy counted the Revolutions of the drive shaft with the Transmission in neutral. Because you were only rotating one side, you'd have to divide by two. Six and a half turns of the drive shaft would be roughly 3.31. Six turns of the drive shaft would be close to 2.73.

So far I've had three cj's, two of which originally came with the AMC 258 i6 / 4.2l and T-5 , and both were spec'd with 2.73 gears.

 

[CJ]

Flyn, we always did it the opposite way. We would jack up one side, and rotate the tire one full Revolution while the other guy counted the Revolutions of the drive shaft with the Transmission in neutral. Because you were only rotating one side, you'd have to divide by two. Six and a half turns of the drive shaft would be roughly 3.31. Six turns of the drive shaft would be close to 2.73.

So far I've had three cj's, two of which originally came with the AMC 258 i6 / 4.2l and T-5 , and both were spec'd with 2.73 gears.

Yes, that does work the same way but ONLY with OPEN differentials.

 

[Flyn'Hawaiian]

Flyn, we always did it the opposite way. We would jack up one side, and rotate the tire one full Revolution while the other guy counted the Revolutions of the drive shaft with the Transmission in neutral. Because you were only rotating one side, you'd have to divide by two. Six and a half turns of the drive shaft would be roughly 3.31. Six turns of the drive shaft would be close to 2.73.

So far I've had three cj's, two of which originally came with the AMC 258 i6 / 4.2l and T-5 , and both were spec'd with 2.73 gears.

You're probably correct. It's been 30 + years since I did that. Thanks

 

[boney]

Flyn, we always did it the opposite way. We would jack up one side, and rotate the tire one full Revolution while the other guy counted the Revolutions of the drive shaft with the Transmission in neutral. Because you were only rotating one side, you'd have to divide by two. Six and a half turns of the drive shaft would be roughly 3.31. Six turns of the drive shaft would be close to 2.73.

So far I've had three cj's, two of which originally came with the AMC 258 i6 / 4.2l and T-5 , and both were spec'd with 2.73 gears.

I find this a curious thing. Someone put a T-5 into my CJ7 (with a AMC 258 i6 / 4.2l and 2.73 gears) and it won't push 5th gear on flat ground with 31 inch tires. Run it in 4th gear? All day long at some pretty good speeds- and that's what it came with, a T-4 .

When I started doing a spreadsheet for T-4 and T-5 and axle ratios and wheel size, I discovered that the gear ratios for the T-4 and 5 are the same, except for the Overdrive 5th gear in the T-5 . The final drive ratio of a T-4 with 2.73 axles (2.73:1 final) is 4% higher than a T-5 with 3.31 axles (2.85:1 final). For comparison; the 2.73 axle and 5th gear in a T-5 computes to 2.34 final drive- at peak torque that puts a stock jeep with 29" tires at 73 MPH.

While my jeep's engine may be well worn, it makes me wonder if the T-4 and 2.73 gears were a pair, and the T-5 and 3.31 gears were a pair from the factory.

Sorry to hijack the thread.

 

[hole]

Mine still has 2.73's (really need to correct that) and started life with AMC 258 i6 / 4.2l /T-5 . A buddy has a couple jeeps, a 7 that started as a AMC 258 i6 / 4.2l /T-5 that was 2.73 and an 8 with a 2.5/T-4 and 3.31's. The handful of CJ folks I've bumped into and got to talking with about this subject, more often than not those with 2.73's had AMC 258 i6 / 4.2l /T4or5 and 2.5 with a T4or5 had 3.31's. By no means an in depth study or survey, just my encounters

 

[Belizean]

Sure glad mine started with 3.54 gears. The T-4 was good. The 4 cylinder was a great little engine and very smooth, but a little under powered if on hills or the interstate. I really like the 3.54 gears with T-176 and the AMC 258 i6 / 4.2l -6 cylinder.