Ok. So after my acts in futility to squeeze all the ponies I can out of the 2.5L I finally concede defeat. I’ll say it. You were all right !!! Ok now that we have that behind us. I’m going to re gear. I’m seeing lots of conflicting posts so forgive my ignorance. I have the 1997 TJ with the 2.5L and 3 speed auto transmission with OD. I’m running 32” tires and will never go over 33’s. It rides the struggle bus hard on the highway. It barely maintains 60 mph and if I allow it to drop into OD it will scarcely maintain 50 mph. The burning question is do I go with 4.56’s or 4.88’s ? I only wanna do this once obviously. I’d like to be able to comfortably run 65 mph on the highway and be able to get out of my own way on the highway. I don’t give a damn about gas mileage. It’s not my daily. Any advice from seasoned guys here would be greatly appreciated!!!
What gears?
- Thread starter Gasman1979
- Start date
Also ! Do I have the Dana 30 and 35 front and rear diffs ? There are no tags at all on either diff. I’m assuming it’s the Dana 30/35 with 3.73 gears.
All 4 cylinder Jeeps in the US have a Dana 35 rear axle. Autos had 3.73 gears, 5 speeds had 4.10.
Do plenty of research and try to find information from people that have done a regear with your same setup. I just re-geared my 2.5, 5 speed with 31’s and went with 4.88, it runs around 3000 RPM’s at 70 MPH in 5th gear. Use this calculator to try different gear ratios for your setup:
http://www.grimmjeeper.com/gears.html
http://www.grimmjeeper.com/gears.html
I'd go with 4.56 for your auto, even with a 4 cylinder. I had a 4.0, 32rh trans, 32s, with 4.56 gears and the rpms were too high IMO for highway cruising speeds. It's your choice.
I’d like to see around 2800 rpm when she hits OD. I think the 4.56 will do the job swell.
Did you fix your running rich issue, yet? The thread you started for that doesn't have a conclusion. Personally, I'd concentrate on that before worrying about gearing or performance gains. Getting it running at the right air/fuel ratio will increase performance, and that may be enough to be noticeable, considering you had problems idling and very high negative long-term fuel trim.
You don't have OD, unless you swapped the trans.
I was thinking the same thing. Some good old fashioned research will serve this project well.
I Have the same setup, 2.5L with the 30rh. I run 33s and regeared to 4.56 and it works well. 4.88 would be too much IMO.
You don't have OD, unless you swapped the trans.
Well, I guess it’s what I’m referring to as OD ??? I can feel it shift through all three gears. If I let up slightly it drops the RPMs by around 500 and I start to lose speed. It’s actually the torque converter locking up
I reckon. It sure feels like an OD. When it locks up the 4 pissy squirrels go on strike and I lose speed 
Yessir I did ! I changed the upstream O2 and the throttle position sensor and the fuel issue went away.
I ordered 4.56’s in Youkon brand. Went ahead and got the whole kit so I can do all the bearings and seals while I’m in there. I like the look and stance with the 32” tires so I think I’ll be happy since that’s the largest tire I will ever run. Today on the highway I was turning 2800ish at 60 mph but when the torque converter locked up and the RPMs dropped to 2200ish it immediately started to lose speed. So I think this gear set will be the answer to my prayers. I just want to be able to run and maintain 65 mph.
There are some good things about the 2.5l/30rh setup despite the lack of power. Its very easy on the drivetrain and you can find and maintain the traction threshold in slippery stuff easy. Really nice combo for running around town too