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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 diff in the rear and open diff 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 and the odd Overland trip and off-roading here and there throughout the year. I am under the impression 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 consisting of long distance driving on the highway to the trail then mild to moderate off-roading.
Do I require new differential carriers to go with the new ring and pinion sets for the increase of gearing from 3.45 to 4.56? Or will my existing differential carriers work with the 4.56 gears as well.
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,
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. I’d say engine speed in this application is actually advantageous, it allows the motor to get up into a rev-range where not only does it make more torque naturally, but the volumetric efficiency improves and BSFC comes down. Jeeps with automatic transmissions will downshift very easily to try to keep the engine in this range, but manual trans versions must be driven like a four-cylinder, rather than trying to lug it like a V8 or a diesel.
That being said, everything that Jeep does revolves around meeting corporate average fuel economy (CAFE) standards in a test environment–not in the real world. So, any aspect of performance that impacts CAFE is hyper optimized for that singular, fuel efficiency goal. Why do I bring that up? Because many people attempt to build with comparing it to stock rpm or stock road characteristics… using OE performance as a baseline results in disappointment time after time.
My recommendation for a manual trans equipped Jeep JL on 35’s is 4.88’s, you can see in the chart below, this works out to just a hair over 2,000 rpm at 60 mph. This is only about 350 rpm higher than stock at cruising speed, but it gets the motor up where it can compensate for the additional wind load, rotating mass, and rolling resistance produced by your modifications.
A lot of people worry about loss of drivability on the highway, or excessive cruising rpm on cross country trips. This really is not the case. In this next chart, to can see rpm vs speed in each gear, and that realistically, even crossing Texas, Montana, or any extended road trip where vehicles typically do 85-90 mph, you’re still below 3,000 rpm, and if the worry is maintaining speed on hills like the continental divide or any challenging ascent, if power is needed, fourth gear is still useable at highway speeds.
You mentioned overlanding, if you plan to build this extremely heavy, and/or tow a tent trailer or the like with it, you may even choose to go deeper than 4.88s, in fact, 5.38s are not uncommon for someone planning to tow on 35s. Looking back to the first chart, you can see at cruise speeds, 5.38s are still only turning 2,200 rpm and change. An additional benefit? The Jeep should actually feel “sporty” if I dare to use “Jeep” and “sporty” in the same sentence.
Hopefully this info helps to clarify some of the misleading internet half-truths.
As far as parts, yes, we offer complete packages for what you need, new carriers are not required, but if looking for traction upgrades they will be needed.
For a daily driver/moderate off roader, I’d lean more toward one of our helical limited slips up front. They operate absolutely seamlessly, and unlike a locker LSDs create zero on-road drivability issues. So, if you foresee the need to travel in adverse–snowy/icy conditions in 4-High, lockers will get you into more trouble than they’ll get you out of.
The two links below will get you everything you need to get the job done, short of fluids, tools, and beverages.
Complete Yukon 4.88 Ring And Pinion Gear Set & Instalation 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
