Question about gears

[wldnrgrs]

Hello. This is my first post to the forum. Just got my MJ that I have wanted for so long. I'm in Georgia. It was a California truck it's whole life. No rust at all. It is a 1991, 4.0L, 5 speed AX15, 4x4, Metric ton package. I am very excited, and have lots of questions. My first question is about the gears. I understand that the truck has a rear Dana 44 and a front Dana 35. The guy that I bought the truck from did a couple of modifications. One was to change the gears. I'm not sure what they were from the factory, but he put 4.56 gears in the front and rear. It has 31" tires. I don't plan to go any larger. Is this a good idea to use this gear setup. It seems to be geared really low. First gear is almost not needed on the street. On the highway, the RPMs read about 3000 at 75 miles per hour. I'm not really an off roader.....yet. the truck will see mostly asphalt. I do like the option though. Would I be better off with a different gear ratio? I am getting about 16 miles per gallon around town? Thanks to whoever responds.

[freakjeep93]

welcome to the club :waving:

first off its a dana 30 not a 35 .

 

and gears area ll about preference. ive never played with them yet but most people agree with 4:10s being just right for 31s. so you might be geared abit high. but revving 3000 at 75 isnt tooooo bad. and you seem to be getting half decent gas milage so idk if youd want to change them or not. if you do keep them incase you ever need/want to go to bigger tires.

someone else will chime in with more info as I'm just spitting out what ive learned from reading here .

[Geonovast]

He's right about your front being a Dana 30. Are you sure the rear is a 44? Dana 35 was a too common rear axle.

 

4.56s are a bit much for only 31s, although it's quite costly to re-gear.

 

Your truck would have had 3.07 gearing from the factory.

 

so you might be geared abit low.

 

Numerically higher ratio = low gearing

[Eagle]

Hello. This is my first post to the forum. Just got my MJ that I have wanted for so long. I'm in Georgia. It was a California truck it's whole life. No rust at all. It is a 1991, 4.0L, 5 speed AX15, 4x4, Metric ton package. I am very excited, and have lots of questions. My first question is about the gears. I understand that the truck has a rear Dana 44 and a front Dana 35. The guy that I bought the truck from did a couple of modifications. One was to change the gears. I'm not sure what they were from the factory, but he put 4.56 gears in the front and rear. It has 31" tires. I don't plan to go any larger. Is this a good idea to use this gear setup. It seems to be geared really low. First gear is almost not needed on the street. On the highway, the RPMs read about 3000 at 75 miles per hour. I'm not really an off roader.....yet. the truck will see mostly asphalt. I do like the option though. Would I be better off with a different gear ratio? I am getting about 16 miles per gallon around town? Thanks to whoever responds.

Your speedometer may be off a little. With 31" tires and 4.56 gears, 75 MPH should be 2906 RPM. I guess that's "about" 3000 RPM, but it's lower than, not higher than.

 

Secondly, for a 1991 the torque peak of the engine is around 4,000 RPM. 3,000 RPM cruise is below the torque peak. That RPM isn't going to harm the engine. You have to remember that the 4.0L engine is an evolution of AMC's older 232 (3.8L) engine that came out in 1964. Back then, nobody had overdrive, and most cars were still manual transmission with "three on the tree." My family owned a number of AMCs from that vintage. They were typically geared with 3.08 axles and small-ish tires, such that the speed to RPM came out to 24 MPH per 1000 RPM. 3000 RPM produced a 72 MPH cruise speed.

 

They ran forever like that. The one we tracked the longest was a 1970 Gremlin that my brother and his wife rallied, autocrossed, and used as a daily driver for four years. When they bought a new '74 Gremlin, he sold it to a local auto parts store to use as a parts delivery runner. Last he heard, it had gone over 375,000 miles and they never had the head off it.

 

My view is that 4.10 gears are the optimum for 31" tires, but if I bought one with 4.56s I certainly would NOT spend the money to change to 4.10s. There would be very little to gain for the expense and trouble. Your 75 MPH cruise would drop to 2614 RPM.

[wannabeMJ]

It is a 1991, 4.0L, 5 speed AX15, 4x4, Metric ton package. àquote]

 

then it should be a 44

[Geonovast]

It is a 1991, 4.0L, 5 speed AX15, 4x4, Metric ton package.

 

then it should be a 44

 

Missed that. Yup, should be 44.

[mvusse]

Like Eagle said, stock gearing would be 3.07. I do disagree on preference for gearing, though.Even though I don't see any justification in regearing what you have, in a stick shift with 31s I would prefer 3.55. 410s would be my preference for 31s with an automatic transmission.

 

Great find, and welcome to the addiction.

[wldnrgrs]

Thanks for the quick responses. That certainly gives me something to chew on. I had been worried about the rpm issue. Thanks for the welcome notes. I look forward to future communications.

[kastein]

Instead of gearing it back closer to stock you should put bigger tires and maybe more lift on it :brows:

[Comanche County]

Slap some 32" or 33" tires on it. I bet your mileage will improve.