Options to help the 42RLE transmission OD issue

[PackFan]

New to Jeeps and the TJ and just discovered the limitations of the 42RLE transmission and it's .69 OD gearing. The Jeep is a 2003 TJ Sahara, 4.0L, 42RLE, 3.73, Dana 44 with Trac-Lok rear, Dana 30 front.

I just installed 31x10.5-15 BFG KO2's that have made the problem much worse. The tranny shifts into OD at around 45 MPH and lags terribly until 65. Mileage has gone in the tank.

Have done a lot of research on the forum. It looks like the 4.0L is most efficient between 2000 and 2400 RPM with a power band between 2400 and 2700. If this is accurate, here are the options that look best to improve results. (Numbers are from the GrimmJeeper calculator with 30.5 entered as the diameter of the KO2's.)

1. Keep the 3.73's- I have just started to routinely hit the OD off button and only turn OD on at 65 plus. In 3rd gear I am turning 2465 RPM's at 60 mph and 2671 at 65. OD on is 1843 at 65 and 1985 at 70.
2. Re-gear to 4.56's- Highway speeds are good with 2253 at 65. My concern is 3rd gear that turns 3014 at 60. Shifting down to pass or pulling a grade (that I do a lot of) could be tough. I would be interested to know the actual experience of anyone running this combination?
3. Re-gear to 4.10's- This may not be worth the effort and expense, but the numbers are improved with 2026 at 65 in OD. 3rd gear is 2710 at 60.
4. Re-gear to 4.27's- This is the sweet spot, but I don't think there is anyway to make them work.

What do you guys think? Also, does the Trac-Lok LSD impact the re-gearing process is any way?

Thanks.

 

[Jramos013]

right now i have a Dana 44 with 3.73 and 33" tires. It does suck at highway but its bareable downtown. From everythin i have read here and other forums, the best choice for 33" and 42rle are 4.88 or 5.13 gears. For 31" most people get 4.88 with an auto.

 

[PackFan]

Thanks Jramos013.

I saw where 4.88's are recommended even for 31's. My concern with 3rd gear performance is even greater than with 4.56's. I am showing that in 3rd gear the 4.88 RPM's would be 3226 at 60 mph. The 4.88's may work great on the freeway, but they may not be ideal for the mountain roads I drive in the Lake Tahoe basin where down shifts are common. Maybe I am wrong, but I just don't see the 4th gear overdrive being able to pull and handle all the twists and turns with either 4.56's or 4.88's.

Is anyone out there running 4.56's or 4.88's with 31 inch tires like the BFG KO2's?

 

[Jamison C]

Thanks Jramos013.

I saw where 4.88's are recommended even for 31's. My concern with 3rd gear performance is even greater than with 4.56's. I am showing that in 3rd gear the 4.88 RPM's would be 3226 at 60 mph. The 4.88's may work great on the freeway, but they may not be ideal for the mountain roads I drive in the Lake Tahoe basin where down shifts are common. Maybe I am wrong, but I just don't see the 4th gear overdrive being able to pull and handle all the twists and turns with either 4.56's or 4.88's.

Is anyone out there running 4.56's or 4.88's with 31 inch tires like the BFG KO2's?

The 4.0 is a long rod, short stroke motor than can handle high revs with ease. 3200rpms is barely breaking a sweat for the 4.0. In fact, that’s typically where it produces peak power/torque.

So many people (myself included when I first started driving the 4.0) view it like a V8 where you never wanna cruise it any higher than 2500rpms for a long period. The 4.0 is an entirely different animal.

When I pull steep hills at 70 I’m usually right at 3200rpms. One of the hills I frequent has me sitting in that range for about 15 minutes or so. I’ve never overheated or had an issue, and I still have passing power to spare to get around semis without cutting off traffic in the left lane. I would also like to throw out that I average 17-18 mpg.

For reference:

(Before someone throws a fit I had my brother snap this photo for referencing reasons like this thread here. I couldn't drive and take a picture this clear at 65mph ).

Don’t be afraid to give the engine some love. If you’re sticking with 31s, 4.88 gears would be the way to go. Switching the OD off on mountain roads and running 3-3.5k rpms wouldn’t hurt it one bit. And you’d have a great low range to work with.

 

[Chris]

Your best option for dealing with the stupid OD ratio of the 42RLE is simple:

Re-gear the thing to the correct gear ratio for your setup, and then get in the habit (and I can't stress this enough) of tuning the overdrive off every single time you start the vehicle.

Do this for a week and it will become second nature. I promise you that if you do this, you will soon forget about the OD issues of the 42RLE and you will be much, much happier with the transmission.

If most people did this, they wouldn't ever look back, and there would be a lot less complaining (I used to be one of the complainers too).

Trust me, make that a habit, and with the proper gears, you'll be golden.

 

[Steel City 06]

To ease your concerns about high RPMs, the stock TJ Rubicon with NSG370 turns 2700 RPM in OD (6th gear) at 70 mph. At 80 it’s basically 3000 RPM. Downshift to 5th and you’re significantly higher. Downshift to 4th and you’re at 4000+ RPM at freeway speeds. I routinely run at 2700+ RPMs all day long (10 hours or more) on the freeway and have no issues with running 4500+ RPM to haul myself up a hill for a few minutes.

You probably don’t want to gear so you’re doing 3000+ in 4th/OD but anything less than that is perfectly acceptable. After I regear to 4.88s I’ll be running 2800+ at freeway speeds in 6th/OD, 3300+ in 5th, and 4200+ in 4th.

Heck, my f150 which has an underpowered V6, 3.55 gears, and a 4 speed OD auto very similar to the 42RLE, I routinely use 2nd gear at 70+ mph to get up hills, which is just below redline.

 

[PackFan]

Chris: I saw you made this suggestion in an old thread too. I have started clicking off the OD at every start-up. It has made a big difference in performance. I have already gone further on this tank than the last, so it has also improved economy.

I didn't realize the 4.0L could rev like this. Thanks for educating me. I guess that puts me at either 4.56's or 4.88's for the re-gear with my 31 inch tires.

Any input on the Trac-Lok? I understand it isn't the best LSD for longevity. The Jeep only has 32K miles so it should still be serviceable. Should I leave it or replace it with a Detroit True Trac while I am in there? Its another $500.00 or so, but "what's another glass of water to a drowning man"?

 

[Steel City 06]

Chris: I saw you made this suggestion in an old thread too. I have started clicking off the OD at every start-up. It has made a big difference in performance. I have already gone further on this tank than the last, so it has also improved economy.

I didn't realize the 4.0L could rev like this. Thanks for educating me. I guess that puts me at either 4.56's or 4.88's for the re-gear with my 31 inch tires.

Any input on the Trac-Lok? I understand it isn't the best LSD for longevity. The Jeep only has 32K miles so it should still be serviceable. Should I leave it or replace it with a Detroit True Trac while I am in there? Its another $500.00 or so, but "what's another glass of water to a drowning man"?

Before you choose your gears, you may want to consider what you plan to do to the Jeep in the future. If you think you’ll stick with 31s or 32s, 4.88 is probably a good ratio. If you think you might add a lift and 33s or larger, you might consider going up to 5.13. 5.13s on 30.5” tires would be a bit unusual, but you’d still be geared taller than a stock manual TJR. The 5.13 would allow you to put those tires on without a 2nd regear in the future.

For the carrier(s), I would recommend a selectable type locker over a LSD or auto locker on a street driven vehicle. Popular options include ARB lockers, the Eaton Elocker4, the older Eaton Elocker, as well as a few others. These will be more expensive (~1k each) than the LSD you’re looking at but they also offer much greater durability and performance over an LSD while still having better street manners than an auto locker. Simply flip a switch to stop the wheelspin.

You can put them in both the front and the rear, however with the rear Dana 35 and/or front Dana 30 you would want to upgrade the shafts eventually to prevent a broken axle shaft, particularly if you go to larger tires.

The Detroit True Trac is a LSD, the Detroit locker is an automatic locker. Both are manufactured by Eaton, along with the electronic lockers. ARB lockers and others are operated by compressed air instead of electric signal and require an air compressor or compressed air storage to operate.

If you think you will want lockers in the front or rear axles at any point I definitely recommend installing them at the time of the regear. Doing lockers and gears in both axles at once will save you ~1k in labor versus doing a regear and then lockers at a later time.

 

[PackFan]

I am not looking at going larger on the tires. 31's will be it. The Jeep is going to be used for running errands around town, exploring trails, and running up to the nearby ski areas. I also plan on towing it behind my motor home and diesel truck on vacations and hunting trips. No rock crawling or significant off trail use is planned.

Based on this use, I wasn't looking at going to full lockers. I thought the Detroit True Trac LSD (or keeping the factory Trak Lok LSD) would be a good compromise. I was going to leave the front open for now. Does this make sense?

 

[Steel City 06]

I am not looking at going larger on the tires. 31's will be it. The Jeep is going to be used for running errands around town, exploring trails, and running up to the nearby ski areas. I also plan on towing it behind my motor home and diesel truck on vacations and hunting trips. No rock crawling or significant off trail use is planned.

Based on this use, I wasn't looking at going to full lockers. I thought the Detroit True Trac LSD (or keeping the factory Trak Lok LSD) would be a good compromise. I was going to leave the front open for now. Does this make sense?

Makes a lot of sense. There is a limit to what an LSD can do, but on gravel and snow packed roads they do great