42RLE, 33" tires and 4.88 gearing

[Marc McKinley]

I recently purchased a 06 TJ Rubicon with the dreaded 42RLE 4 speed auto transmission. I plan to install the Old Man Emu Ultimate lift kit from DPG Off-Road and wanting to run 33" BFG tires.

With this set up I plan to re-gear to 4.88. My question is what kind of RPM's can I expect at 55mph and 70mph? I do most of my commuting at around 55mph. I'm hoping at 55mph my RPM's will be around 2100 and at 70mph around 2500 RPM.

4.88 seems very deep but everything I've read points to this or even deeper gearing. I don't want to go deeper as this is going to be my daily commuter and I want to stay in the 2000-2500 range most of the time. With my stock 4.10 i'm running right at 2000RPM when going 55mph and I have no complaints.

Any experience with this set up and actual RPM stats will be greatly appreciated!

****I plan to use Revolution ring and pinion gears, where is the most reasonable place to purchase****

 

[Jerry Bransford]

Go with 5.13 for your 42RLE and 33's, 4.88 is only kinda-sorta ok with 33's. 5.13 would be a more optimal ratio, especially on the highway. Don't even give the rpms a second thought, they will be optimal and not too high for the highway. I'm running the next lower ratio, 5.38, for 35's and even its rpms are a tad lower than would be optimal on the highway.

 

[Marc McKinley]

Go with 5.13 for your 42RLE and 33's, 4.88 is only kinda-sorta ok with 33's. 5.13 would be a more optimal ratio, especially on the highway. Don't even give the rpms a second thought, they will be optimal and not too high for the highway. I'm running the next lower ratio, 5.38, for 35's and even its rpms are a tad lower than would be optimal on the highway.

What kind of RPM's do you think it would be turning with 5.13 at 55-60mph?

 

[Jerry Bransford]

With 5.13, 33" tires and 42RLE your rpms would only be 2000 at 55 and 2150 at 60 mph. Barely enough for the highway so even with 5.13 I'd likely be shutting the Overdrive off to get the rpms back up into a more efficient range.

 

[psrivats]

I would also recommend 5.13s for 33" tires.

I have 4.88s and 32" (265/75R16) and I get ~2000 RPM at 55 and ~2500 at 70 which is good on the hwy. With 42RLE + 33s and 5.13s you'll be nearly the same.

 

[LukesfirstJeep]

I would also recommend 5.13s for 33" tires.

I have 4.88s and 32" (265/70R16) and I get ~2000 RPM at 55 and ~2500 at 70 which is good on the hwy. With 42RLE + 33s and 5.13s you'll be nearly the same.

+1

I have 4.88 and 32's. I had 3.73 gearing before and was worried I was going too deep. 4.88 is not too deep. The OD on the 42RLE transmission really drops the RPM's

 

[Mr. Bills]

For rpm's at various speeds with particular gear ratios and tire sizes use this calculator:

http://www.grimmjeeper.com/gears.html

 

[Marc McKinley]

l would love to hear some real word data from somebody that has the 42RLE trans, 4.88 or 5.13 and running 33" tires. Actual speeds with corresponding RPMs.

Re-gearing isn't cheap so I want to be 100% positive and do it right the first time.

 

[Jerry Bransford]

l would love to hear some real word data from somebody that has the 42RLE trans, 4.88 or 5.13 and running 33" tires. Actual speeds with corresponding RPMs.

Re-gearing isn't cheap so I want to be 100% positive and do it right the first time.

You were given the rpms you would see at the speeds you asked about for the recommended 5.13 ratio. If you didn't believe or like them use the above linked-to calculator to verify them.

And not that 5.13 is overgearing for 33's at all when you have a 42RLE transmission, not even close, it's far better to be slightly overgeared than slightly undergeared. That 5.13 recommendation was made with the highway in mind, not offroad use where it would also be superb.

 

[2006TJ1]

I have 4.88 gears with the 42RLE and 33s. It works fine, though it sucks around 45-50 mph when the OD kicks. The 42RLE loves OD and will drop into it ASAP so that must be taken into account. When I am on the highway though, I am typically going 65+ so this isn't an issue. When I am putting around town going anywhere from 35-55mph i just flip OD off and don't think about it. I don't think you could go wrong with 5.13s or 4.88 gears.

Edit: I should also add that when deciding my gearing, I did consider tire size. I use my jeep mostly on road about 60 highway/40 around town, and so I knew I do not plan on going up in tire size. I will staying at 33s or testing out some skinny 32s just to experiment. 5.13s would leave me overgeared for 32s. 4.88 puts me in the position to be geared both for 33s and 32 should I ever change.

 

[Marc McKinley]

You were given the rpms you would see at the speeds you asked about for the recommended 5.13 ratio. If you didn't believe or like them use the above linked-to calculator to verify them.

And not that 5.13 is overgearing for 33's at all when you have a 42RLE transmission, not even close, it's far better to be slightly overgeared than slightly undergeared. That 5.13 recommendation was made with the highway in mind, not offroad use where it would also be superb at.

I highly regard your opinion and knowledge. I have read many of your posts on this forum and would definitely consider you one of the most informed on the TJ platform. I just haven't heard from anyone running exactly what I'm wanting. Everyone is speculating with 32" or 35" tires. I tried to plug in the information on the Grim Jeeper site but couldn't get it to work I'll have to mess with it when I have a little more time.

I truly appreciate everyone's input!

 

[Brianj5600]

Here are the numbers for 4.88 and 5.13.

 

[2006TJ1]

Here are the numbers for 4.88 and 5.13.

View attachment 117442

Keep in mind a tire with 33" on the side wall is only ever about 32.5" tall, and is likely only 32" when loaded and at street PSI. Real RPMs will be higher than the calculator might predict when using 33" as the tire size because these factors are not considered.
It is not a big change, but it may be worth while to look at RPMs when plugging in 32.5" or 32" to represent double the rolling radius.

 

[S.McArthur]

I have 5.13/42RLE/33” tires, at 70mph I’m at ~2600rpm. My fuel mileage got better by about 1-2MPG and the Jeep just drives better all around.
I can run 80mph around ~2800/2900 without issue. I never turn my OD off, drives perfectly.
I had revolution gears installed and the installer had never used them and said they set up easy and he is now adding them to his recommendations for gears.
Edit: Tires in the pic are 33x12.50, I’ve since replaced them with BFG KO2, 285/70R17

 

[Mr. Bills]

l would love to hear some real word data from somebody that has the 42RLE trans, 4.88 or 5.13 and running 33" tires. Actual speeds with corresponding RPMs.

Re-gearing isn't cheap so I want to be 100% positive and do it right the first time.

Unfortunately you can never be 100% positive, but you can have a high level of confidence in your choice.

The actual rpm's at a given speed for a particular gear and tire combination will vary depending upon the particular tire being used and to some extent the air pressure employed. This is because tire diameters vary slightly between tire manufacturers for the same size marked on the sidewall. For this reason, "real world" numbers that may be reported on this or any other forum will always vary from your own "real world" results unless that other person is using the exact same tire at the exact same tire pressure on a jeep with the exact same weight.

The only way you can come close to being "100% positive" is to use the rolling radius of your 33" tires mounted with the vehicle on the ground, then double that number and use the sum as the diameter in the gear calculator rather than use the published diameter of the tire. (Rolling radius is the distance from the center of the hub to the ground.)

Also, everyone has a different opinion as to the "optimum" rpm's at cruising speed. Some think 3000 rpm at 75 mph is perfect, others find perfection at 2700 rpm. (My Grimm Jeeper calculations are 2704 rpm's at 75 mph using 5.38 gears and 315/75R16 tires, commonly referred to as metric 35's, which have a typical diameter of 34.6". If I recall correctly, my "real world" observation is 2750 rpm at 75 mph.)

Your best choices for the 4.0 I-6, 42RLE transmission and 33" tires will be either 4.88:1 or 5.13:1.

The Grimm Jeeper calculated rpms at 75 mph for a true 33" tire with 4.88 gears is 2571. The rpm's at 75 mph with 5.13 gears would be 2703. Based on my "real world" observations with almost identical calculated rpm's at 75 mph I would choose the 5.13 ratio.

However, if you are running metric 33's such as 285/75R16, those typically have published diameters of 32.8". The calculated results for rpm's at 75 mph would be 2666 with 4.88 gears and 2802 with 5.13 gears. I would still choose 5.13.

Best of luck with your decision.

 

[Old Man TJ]

Go with 5.13 for your 42RLE and 33's, 4.88 is only kinda-sorta ok with 33's. 5.13 would be a more optimal ratio, especially on the highway. Don't even give the rpms a second thought, they will be optimal and not too high for the highway. I'm running the next lower ratio, 5.38, for 35's and even its rpms are a tad lower than would be optimal on the highway.

Hey Jerry,

I just wanted to say thank you for all the advise over the years. I have a 2004 Wrangler X with the 42rle, I always look for your advice on all things TJ. and now that I am thinking about going to 33's from 31's and knowing how much of a lug the 3.73 gears are with 31's, I looked for your experience.

Thanks again!