Tales From The Tech Line: Re-Gearing A Jeep JLU Wrangler With 35-Inch Tires

Q-

Hello Yukon Team,

I have a 2021 Jeep Wrangler JLU Sport S. I was able to retrieve my build sheet based on VIN and I have a M186 front axle and M220 rear axle with stock 3.45 gearing. I have a limited slip differential in the rear and an open differential in the front. I recently completed a 3.5-inch suspension lift and added 35 x 12.5R17 tires. As you probably are aware, this has made the Jeep pretty gutless. I should also mention I have a six-speed manual with the 3.6-liter Pentastar V6 engine. My goal is to use the Jeep as a daily driver with occasional overland trips and off-roading throughout the year. I am under the impression that 4.56 gears are a good fit for my application.

My questions for your team are:

• Is a 4.56 gear set right for my application, keeping in mind it’s a daily driver with overlanding that consists of long-distance highway driving to the trail and mild to moderate off-roading?
• Do I require new differential carriers to go with the new ring and pinion sets when increasing gearing from 3.45 to 4.56, or will my existing differential carriers work?
• Do you make a locker that fits a front M186 axle?
• Do you make a 4.56 ring and pinion gear set for both the M186 and M220?

I appreciate your time and guidance on these newbie questions. I look forward to your response and any further knowledge or guidance you can share with me.

Regards,

Brandon Williams

A-

Hey Brandon,

You are correct, the 3.6-liter “minivan” motor is no torque monster by any stretch of the imagination, but it does run fairly well when allowed to rev a little. Engine speed in this application is actually advantageous, as it allows the motor to operate in a rev range where it produces more torque, volumetric efficiency improves, and brake-specific fuel consumption comes down. Jeeps with automatic transmissions will downshift easily to stay in this range, but manual transmission versions must be driven more like a four-cylinder rather than lugged like a V8 or diesel.

That being said, everything Jeep does revolves around meeting corporate average fuel economy (CAFE) standards in a test environment, not the real world. As a result, performance is optimized for fuel efficiency rather than real-world drivability. Using stock RPM or road characteristics as a baseline often leads to disappointment.

My recommendation for a manual transmission Jeep JL on 35-inch tires is 4.88 gears. As shown in the chart below, this results in just over 2,000 RPM at 60 mph, only about 350 RPM higher than stock cruising speed. This places the engine in a range where it can better compensate for added wind load, rotating mass, and rolling resistance from your modifications.

Many people worry about highway drivability or excessive cruising RPM on long trips. In the next chart, you can see RPM versus speed in each gear. Even at sustained speeds of 85–90 mph, engine speed remains under 3,000 RPM. For climbs such as the Continental Divide or other long ascents, fourth gear remains usable at highway speeds when additional power is needed.

Regarding overlanding, if you plan to build the Jeep very heavy or tow a tent trailer, going deeper than 4.88 gears may make sense. Gear ratios like 5.38 are not uncommon for towing on 35-inch tires. Even then, cruising RPM remains reasonable at just over 2,200 RPM. An added benefit is that the Jeep will feel noticeably more responsive.

Hopefully this information helps clarify some of the misleading half-truths found online.

As far as parts go, we offer complete packages for what you need. New carriers are not required for the gear swap, but they will be needed if you decide to add traction upgrades.

For a daily driver and moderate off-road use, we generally recommend a helical limited slip in the front. These operate seamlessly and create no on-road drivability issues. In snowy or icy conditions when using 4-High, lockers can actually create more problems than they solve.

The two links below include everything you need to complete the job, aside from fluids, tools, and beverages.

Complete Yukon 4.88 Ring and Pinion Gear Set & Installation Kit

USA Standard Gear Spartan Gear-Driven LSD for Dana 30

Feel free to reach back out if you have any further questions.

Gus Pyeatt, Tech Support Specialist