The 1-Millionth Gearing Question Ever
I have a question about gearing my '95 2.5L Wrangler with the stock 4.10s. I am running 33x12.50 tires right now. I do have some engine mods which help to push the tires a wee bit, but I still can't use Fifth gear. I have been contemplating regearing, but I don't know much about it. I like to trail through the woods and play in the mud and am not so much a rock crawler. It's also my grocery getter and I run the highway on occasion. What would be a good gear to run? My friends have recommended 3.73s, but your article "Gearing for Power" seems to say otherwise. I would like to gain Fifth gear back.
Mike Cooper, Carlisle, Pennsylvania
Honestly, 33s and a 2.5L suck. Your friends are dead-wrong. With the stock axles, 4.88s are the lowest you can go. If you're keeping the stock axles, put in 4.88s. If you're swapping axles and they'll take it, go to 5.13s or 5.38s if you want to keep the 33s and still use Fifth gear.
I've got an '89 Wrangler with the 2.5L. I used to run the stock 4.10 gears with 29- and 30-inch tires without too much trouble. I then went to 31x11.50-15 Swamper LTBs and it got pretty gutless. I'm now running 4.88s in the stock axles and, although I still downshift on big hills from Fifth to Fourth, I'm pretty happy with the performance from the stock engine.
As for 3.73s, you'll lose a lot of off-the-line acceleration and ability to maintain speed. Gearing is mechanical advantage. A 2.5L isn't a very strong engine and doesn't make a lot of torque. To use a bicycle as an analogy, if you've ever peddled a mountain bike or 10-speed up a big hill, you'll know. Go on the small gear on the rear sprocket (like the 3.73) and you're spinning fewer revolutions, but it takes more force from your legs to push the pedals down. Go to the large gear on the rear sprocket (like the 4.88s or 5.13s) and you're spinning more revolutions, but it takes less leg force to get up the hill. The 2.5L just doesn't have the oomph to pull the 3.73 gear. While you may find yourself shifting from Fifth to Fourth frequently with the 4.10s, I think you'll find yourself shifting from Fourth to Third with the 3.73s. With that tire size, the 3.73s are better-suited to a 4.0L six or a small V-8.
Fact-Finder
Let me start by telling you that I think a lot of the info in your magazine is over my head. I am a Jeep guy from way back, but there is a lot I don't know about the world of custom suspensions, axles, and gears. I currently drive a '99 TJ Sahara. I have added a 4-inch Skyjacker lift and 33x12.50R15 BFG ATs. I have had the Jeep for ten years, but only started tinkering with it in the past year. I know it is a 4.0L six, and an AX-15 transmission. I do not know the particulars on the T-case, axles, or the gears.
Justin Nelms, Moultrie, Georgia
Your T-case is a New Venture Gear (NVG) 231J, commonly known as a NVG231. Behind the 4.0L engine it's got a 23-spline input gear. The 2.5L four-cylinder models got a smaller 21-spline input gear. The low-range ratio is 2.72:1 which is provided by a three-pinion planetary gearset, and it runs a 1-inch wide chain inside. There are components available to upgrade it with a 32-spline slip-yoke eliminator, a stronger six-pinion planetary gearset, and wider 11/4-inch chain and front output shaft.
Your front axle is a low-pinion Dana 30 with 27-spline shafts and Dana 44-sized Spicer 297/760-series U-joints. The rear axle on most TJs is usually a Dana 35 with 27-spline C-clip axles, however it's not unusual to find the much better Dana 44 axle with non-C-clip, 30-spline shafts in Sahara models of this era. This link should help you identify your rear axle based on the diff cover shape: drivetrainspecialists.com/categories/dana/.
With the exception of the Rubicon models, TJs with the 4.0L engine were commonly offered with 3.07, 3.55, or 3.73 axle gears. I'd wager you have 3.55s. Look for a stamped sheetmetal tag on either the front or rear axles held on by one of the diff cover bolts. This tag should have the gear ratio stamped on it. The front differential will be open. The rear differential could be a Trac-Lok limited-slip or an open diff.
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