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Nuts & Bolts: Lower Gearing

Posted in How To: Tech Qa on February 8, 2019
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I have a YJ with a 2.5L engine, an AX-5 transmission, and an NP231 with a 4:1 Tera-Low kit, 4.88 axle gears, and 37-inch tires. What is the best way to get lower gearing for rockcrawling while still staying streetable? Budget is always a concern.
Joe F.

We’re a little surprised that your combination isn’t enough for your needs already, but extreme situations and an anemic four-cylinder could cause you to run out of gearing. Your AX-5 transmission has a 3.92:1 First gear, so with the Tera-Low kit and the 4.88 axle gears your overall crawl ratio is a very respectable 76.5:1. A lot of guys run less than that with success, but that’s usually with a bigger engine. The 2.5L isn’t exactly a torque monster, so it needs all the help it can get from gearing to stay running in tight circumstances. As you probably know, you’re already pretty well maxed out with your existing drivetrain components, so the only options for deeper gearing are going to involve some extra work.

One direction to go would be to swap in an engine with more torque. A Jeep 4.0L and a Chevy 4.3L are good choices, but any engine swap will require adapters, wiring, fuel injection modifications, and much more. We would avoid running a V-8 with the Tera-Low kit. You could also opt for an aftermarket transfer case like an Atlas from Advance Adapters ( with a 5:1 low range, which would drop your crawl ratio to 95.5:1. However, neither the engine nor the transfer case swap is particularly budget friendly.

Perhaps the least expensive option, but not necessarily the easiest, would be to replace the AX5 with an SM420 four-speed transmission. Technical Editor Verne Simons did this swap several years ago, and the article offers quite a few details that would be useful to you (“Granny Gearing,” Jp magazine, Nov. 2014; Used in the 1950s and 1960s in a wide variety of GM applications, the transmission has a whopping 7.02:1 non-synchronized First gear. They are getting a bit difficult to find, but they are still out there if you do some digging. An SM420 would drop your crawl ratio to roughly 137:1, which is more than enough for just about anything. Through an odd combination of circumstances, the 2.5L Jeep engine shares an engine bolt pattern with the GM 2.8L 60-degree V-6 that was used in various cars and S-10 trucks in the mid 1980s. Quite a few of these came with manual transmissions. The bellhousing from a longitudinally mounted 2.8L manual tranny is compatible with the SM420 and only requires drilling and tapping two holes. As a result, you can mate an SM420 to a Jeep 2.5L with factory junkyard-sourced parts. You’ll need a crank position sensor adapter, a different clutch, a special throw-out bearing and pivot ball, and a slave cylinder with a hose adapter. On the transfer case side, you’ll need an aftermarket adapter to mate the SM420 to your existing NP231, which is not cheap but is readily available through Novak Conversions ( and Advance Adapters ( Since you’ll already be pretty deep into the swap, you might consider swapping the 21-spline input gear of your transfer case for a much stronger 23-spline unit. With this combination you actually gain some rear driveshaft length and a whole bunch of additional gearing while avoiding the added hassle and expense of an engine or transfer case swap. The one downside is that you’d lose Overdrive, but you should still be able to maintain reasonable highway speeds with 4.88s and 37s.

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