Rubicon Regearing Help

[nomadjay]

I just put 4.88s in my 2019 JLUR (3.6L with 8sp auto). I'm running 35s and hated the way the Jeep drove on the 4.10s.

The 4.88s are probably too much for 35s. I'd guess they are perfect for the 37s.

As far as gas mileage, you aren't going to get a lot back. My mileage is basically unchanged from 4.10s to 4.88s. But the Jeep drives so much better now. I'll turn about 2300 rpms in 8th running 70mph down the highway. The Jeep no longer seems to be "working" to maintain speed.

Thanks for this! What MPG are you getting roughly? I know I’m probably losing a bunch just because of the extra weight, but Seeing 8th again would be great, plus also not feeling like the jeeps working so hard would be amazing.

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[nomadjay]

If you have 37s now but there is a chance of going down to 35s in the future, go with 4.88. If you keep 37s and live at high altitude and live where there are a lot of hills, I would consider 5.13.

Makes sense, thanks for the tip! My time is kind of split between high altitude and low as I move around through the year. But leaning towards dropping down to 35s then regearing to 4.88. Hoping that strikes a good balance to deal with the extra cargo weight on the jeep.

 

[shane h.]

Thanks for this! What MPG are you getting roughly? I know I’m probably losing a bunch just because of the extra weight, but Seeing 8th again would be great, plus also not feeling like the jeeps working so hard would be amazing.

Saturday I made a 140 mile run to finish up the 500 miles on the gears before the gear check. Going up north into a hard north wind I got just over 14mpg at posted speeds. Coming home had that wind as a tailwind and got 18.5mpg at posted speeds. I was getting 14mpg across the board on the 4.10s.

 

[nomadjay]

Saturday I made a 140 mile run to finish up the 500 miles on the gears before the gear check. Going up north into a hard north wind I got just over 14mpg at posted speeds. Coming home had that wind as a tailwind and got 18.5mpg at posted speeds. I was getting 14mpg across the board on the 4.10s.

18 would be the dream! I’m averaging around 13-15 on the highway. 🫠 Going to drop to 35s and do the regear. Will post a follow up once I get it all sorted. Thanks for the help!

 

[wibornz]

If you never plan on pulling a camper, 4.88 gears for 37 inch tires. If you plan on running 37s, 40s or towing a camper. Defiantly do 5.13 gears.

 

[A Sober Animal]

You're not likely to get back a ton of mileage with a regear. Maybe a little bit don't hold your breath.

Best bet for MPG is to keep it at about 65MPH or under. Wind resistance increases as the square of your speed. So going 70MPH, you have 4x the wind resistance of 35MPH.

Smaller tires help aerodynamics. Removing stuff from the outside of the Jeep helps too. But that conflicts with what makes it good for overlanding.

For most people, 4.88 or 5.13 are the best ratios for 37s. For overlanding with a lot of highway miles, 4.88 is probably the better choice. You'll get full use of 8th gear and still have plenty of gear for technical off road trails.

I read your write-up on your website around regearing - is the only drawback to going to a taller gear (stock 4.10->4.88) going to be my crawling performance, given the tires will move slightly faster? If so, other than the cost, I see no reason not to do this. I'm hoping it will also help reduce stress on the engine a bit - I want to keep this vehicle forever, so starting to look at things I can do to help with longevity.

Saturday I made a 140 mile run to finish up the 500 miles on the gears before the gear check. Going up north into a hard north wind I got just over 14mpg at posted speeds. Coming home had that wind as a tailwind and got 18.5mpg at posted speeds. I was getting 14mpg across the board on the 4.10s.

I've been on 37s now for about 15k miles without a regear and am now debating it. I drive out to Denver each year for a wheeling trip and it always feels like the vehicle is working to keep speed, and my MPGs have tanked (12-14mpg regardless of scenario). Especially at altitude, I've felt like the engine is working really hard. I've also had issues making it up the mountain at speed (trying to maintain 75mph+ will result it in overheating).

 

[shane h.]

I've been on 37s now for about 15k miles without a regear and am now debating it. I drive out to Denver each year for a wheeling trip and it always feels like the vehicle is working to keep speed, and my MPGs have tanked (12-14mpg regardless of scenario). Especially at altitude, I've felt like the engine is working really hard. I've also had issues making it up the mountain at speed (trying to maintain 75mph+ will result it in overheating).

Look at the RPM Extreme fan controller. It's a little pricey but I wish I had put one in both of our Jeeps the day we brought them home from the dealer. My jeep coolant temp hasn't been over 201* since install. I'm really curious to see how well it works when we head up to the San Juans next month.

 

[EbyCreek]

I put one on my 392 Wrangler and everything runs about 25 degrees cooler. I also wish I would of I installed this earlier.

 

[grimmjeeper]

I read your write-up on your website around regearing - is the only drawback to going to a taller gear (stock 4.10->4.88) going to be my crawling performance, given the tires will move slightly faster? If so, other than the cost, I see no reason not to do this. I'm hoping it will also help reduce stress on the engine a bit - I want to keep this vehicle forever, so starting to look at things I can do to help with longevity.

4.88 is a shorter gear than 4.10.

 

[Jeep Wick]

Look at the RPM Extreme fan controller. It's a little pricey but I wish I had put one in both of our Jeeps the day we brought them home from the dealer. My jeep coolant temp hasn't been over 201* since install. I'm really curious to see how well it works when we head up to the San Juans next month.

Running the a/c also cranks up the fan and drops running temps in my experience. Free mod.

 

[Bobby Hank]

Probably true, but with 4.10 you're always starting off from a stop in like second or third gear compared to having 4.88 or 5.13 gears. 4.88 or 5.13 gears makes it easier driving in town, stop and go traffic, and ability to go slower in technical off road situations.

And you'll still have 8th gear for highway driving.

I have the 4:10 and have never started from a stop in anything but 1st. I watch the gears.
Also, on highways, I have to switch to manual frequently to keep it in 8th on even slight uphills.

 

[ArmyRN]

I have the 4:10 and have never started from a stop in anything but 1st. I watch the gears.
Also, on highways, I have to switch to manual frequently to keep it in 8th on even slight uphills.

Yes, technically you're starting off in first. But if you geared it lower, your current first gear (mechanically) would be like 2nd or 3rd in a properly geared Jeep. Meaning you'd have a lower gear or two (mechanically) before getting to your current first gear.

Does that make sense?

The tranny's first gear ratio is the same in both our Jeeps. But mine having lower gears in the diffs than you, if we both have 35" tires, my "first gear" is lower than yours. And I don't have to manually shift on the highway.

 

[A Sober Animal]

4.88 is a shorter gear than 4.10.

Look, I said I read it; I didn't say I fully understood it . In all seriousness though, my original questions still stand - is the only drawback to a regear slightly decreased crawling capability? And will this help reduce stress on the engine/vehicle?

 

[grimmjeeper]

Look, I said I read it; I didn't say I fully understood it . In all seriousness though, my original questions still stand - is the only drawback to a regear slightly decreased crawling capability? And will this help reduce stress on the engine/vehicle?

Too many variables to give a simple answer, honestly.

Part of it is that you ease the stress on one set of parts and transfer it to another. But if you're within spec on how much overall stress is on parts, the effect is neglible.

Going shorter on the gears (higher number) reduces the stress on the transmission, transfer case, and driveshafts. But it transfers it to the differential/locker and the axle shafts. But they're rated for a fair bit of abuse (to a point).

That being said, I've seen people break stuff with stock tires and gears. It's all in how and where you drive it.

 

[UTME]

I have 35” ish tires with 4.56 ratio and get about 16 mpg. Works good for daily driving and occasional off roading.

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