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37" regear 4.88 or 5.13?

16K views 42 replies 11 participants last post by  Asphalt Assault 
#1 · (Edited by Moderator)
I have an '07 JK 4dr Unlimited X automatic. I have 37x13.50R17s on it, and am looking to regear. I believe it comes with 4.11s, and I'm looking at 4.88s, I don't know if 5.13s might be better.

All the threads I've read have been about manuals, and the automatic should have a different OD ratio. Right now, I'm at about 2,800 rpms, going 75mph on the hwy. I don't want my gear ratio so high that I can't go over 60mph.

Looking for advice and help on someone that's done it... with an automatic.

Thanks.

Also.... checking prices, the average is about $1,500 to have them regeared... that's including parts. So... about $1k of labor. I'm mechanically inclined, never changed a transmission, but the usual crap, oil, tires, brakes, none of which is remotely hard. So I'm wondering if doing the regearing yourself is possible, or if that's just a nightmare waiting to happen. I don't mind difficult and complicated, I just don't want to eff something up and strip my gears, have to buy a new axle, etc. If it is possible, how many hours of work did it take you?
 
#2 ·
I would say go with the 5.13 gears. But something doesn't add up with your RPMs. My auto transmission 4dr Unlimited has 5.38 gears and 37" tires, and I'm at 2600 RPMs, going 75 mph on the hwy. For you to be at 2800 RPMs with stock gears, you must be in third gear. That would make sense though, because I believe your Unlimited X has 3.73 gears in it, and my Unlimited Rubicon had 4.10 gears, and with stock gears mine was at 3000 RPMs at 75 mph, ... in third gear.

I paid $900 for parts and labor for my gear install. Teraflex was advertising $999 for parts and labor a few months ago. I think better bargains than $1500 can be found. I would not attempt a gear change myself. You need a few special tools to do the job. I guess you could rent them someplace, but I still wouldn't do it. Just my .02
 
#7 ·
I had 3.73s with 35" Duratracs (315/70/17),went to 5.13s thru Allens Offroad $710. including the new front carrier...paid $775 for labor at The Car Guys in Brighton ,Mi......could not be happier with my decision.More get up and go,better shift points,and so far,better fuel mileage..hope this helps
 
#8 ·
In 07 4.10 was an option in non Rubicons so he could have 4.10 in his X. Wish they still offered it as an option.

I like the idea of 4.88 better. I think many go to high when regearing but since you are running 37's, you can get by with them.
 
#9 ·
What's the biggest downside to gearing too high? Just low top speed? Engine wear through high RPMs without enough resistance? I've seen a lot of people say they put 4.88 on their 33"s and 35"s, and loved it.

Really what I'm looking for is better gas mileage. With 37" XTs and 4x4, I crawl over everything.

Also, if someone reads this comment too. Doesn't lifting your vehicle make you lose torque? I have a 2" lift, and I want to put Flared Fenders, so I can lower it back down 2", and the 37"s will still have clearance- the wheel wells themselves are huge. I have a lot of friends who drive pickups and diesels, and they say when they lift theirs, they lose a lot of torque in take off, and towing power. If I lower it, will I get more torque back?
 
#10 ·
Lowering your Jeep will decrease wind resistance on the highway, and improve gas mileage, but it wont increase torque. Lower gearing (higher ratio numerically) will improve torque. Your friends did not lose torque because they lifted their trucks, they lost torque because they lifted their trucks and installed bigger tires, which effectively decreased their gear ratio. They could regain the lost torque by gearing their trucks lower (increasing the gear ratio) to compensate for the bigger tires.

One thing you need to worry about is not gearing it low enough. Right now you can turn OD off and drive down the freeway at 2800 RPM, and that is fairly easy on the engine, and could be done all day long if you want. With decently low gears (5.13s) you will be able to stay in overdrive and your Jeep will be running at about 2500 RPM. If you choose a gearset halfway between your stock gears and an adequately low set of gears, you may end up with a situation where the gears are still not low enough for your Jeep to be able to stay in overdrive, but they are too low for your Jeep to be able to drive comfortably in third gear. In other words, too tall geared for OD, too low geared for 3rd, and constantly shifting between third and fourth gear. 4.88s are right in the middle of where it should be with 37" tires, and where you are now. I had a Jeep with 4.88s and 37s, and it was horrible. My current Jeep with 5.38s and 37s is much better. Even my Jeep could stand to be geared lower for highway use though. If they made them, 5.67s would be perfect for highway use with 37" tires. And you won't have a low top speed because of the gear changes. If your Jeep had enough power, and did not have a speed limiter, you would hit redline at 160 mph with those 37" tires and stock gears. With 5.13 gears your top speed would drop to something like 140 mph.
 
#13 ·
wendelb said:
Really what I'm looking for is better gas mileage. With 37" XTs and 4x4, I crawl over everything.

I have a 2" lift, and I want to put Flared Fenders, so I can lower it back down 2", and the 37"s will still have clearance- the wheel wells themselves are huge.
I'm not saying I don't believe you, but having the ability to crawl over everything with 37s and a 2" lift, is a little hard to picture. I would think your stock fenders would bust off with just moderate articulation.

We need some pictures :popcorn:
 
#23 ·
I'm not saying I don't believe you, but having the ability to crawl over everything with 37s and a 2" lift, is a little hard to picture. I would think your stock fenders would bust off with just moderate articulation.

We need some pictures :popcorn:
you know its not all about lift. lets look at mathamatically for a second..... 32"s to 37"s is 5" difference divided by 2 = 2.5"s. to maintain the same tire fender gap you need just 2.5"s lift. now if you set it up right you can have a good amount of droop. a great set up for rocrawling as long as you have the break over clearence. if you get skids and do the 1.75" tummy tuck then that is like 3.5" of lift or like getting a set of 40"s under that jeep rather than just 37"s. this is a great LCOG plan. good job.
 
#15 ·
I've had it for a year, so I haven't done much too it, which is disappointing. I'm about to put $5k into it, triple loop brush guard, offroad lights, pro air intake, exhaust headers, dual catback exhaust, ecu tuner, to diamond plating, rear bumper/tow with oversized tire carrier, and a winch... the works. Just been out-o country, and I'm a DIYer. In 3-4 months, I'll have a month off, and I'll get it done. So for now, here's what it looks like.







That's going down the devil's staircase in Sedona, AZ... the picture doesn't do it justice- getting up it was the easy part, going down it without braking an axle was the difficult part.
 
#18 ·
Actually all the pics are "low quality" so mabey not. Just brings back memories, this guy doesnt have a point to prove atleast. If hes a movie producer or rockstar than I guess we will have to worry.

He does have a month off, wonder what thats all about??

Sorry guy just having fun with you..
 
#20 ·
Lift and wheels that's all that's done with it so far. I actually checked my RPMs today. On the hwy, in 4th, going 60, I'm at 2,100. At 75mph, I'm at 2,500-2,600, in 4th, and at 75 in 3rd, I'm at 3,900rpms. I didn't check 3rd at 60, because I got to my exit. "Arthur" was very good btw, I recommend seeing it.

The last picture sucks so much, because that's actually a pulled frame from the video of me going down it. And the video sucks because my wife recorded it, and half of the video she was pointing at the ground, the trees, or the sky, in between that she's screaming at how I should be driving.... so I opted out of posting the video.... a stillshot action photo was better.

John: I have a month off because I do private security contracts conus and oconus. 75-90 days on, usually 30 days off, plus comp if medical recovery is granted. So I go 3 months without spending any of my paychecks, beyond mortgage, and have thousands to squander in toys and hobbies during my breaks.... but I buy a lot of guns, and toys for guns, so the jeep has been neglected. Time for me to make it up to it!
 
#22 ·
Lift and wheels that's all that's done with it so far. I actually checked my RPMs today. On the hwy, in 4th, going 60, I'm at 2,100. At 75mph, I'm at 2,500-2,600, in 4th, and at 75 in 3rd, I'm at 3,900rpms.
Did you buy your Jeep used, with the lift and tires already on it? It has already been re-geared. I'm betting it has 5.13s in it.

Nice looking Jeep and family, ... congrats!
 
#21 ·
She's not standing, you lower the back seats, to unlock the head rest, then bungee the headrest down, and lift the seat back up.... so people in back, sit on the top of the back seat, without the headrest in the way, and are leaning against the rear roll bar.... bracing their arms with the middle roll bar. there is a second lady in the back though, that couldn't hold on, and fell down into the jeep. However, if I was going to alter a picture, I'd pick a grander scheme than driving down a bumpy hill... just sayin....
 
#24 ·
Bought it used in '09, had 35"s on it, with barely any tread left, with the lift-gave the 35" spare to a friend's 4-runner, and got 37"s. It might have 4.88, but I would find it hard to believe someone put 5.13s in it, with 35"s. Other than comparing speed to RPMs, and using statistics, how would I find out what gear I have in it, without just taking apart the diffs?

MOP: I notice your tag says 37"s with 5.38s- what RPMs are you at? What's your gas mileage- Mine blows, I'm at 13.5mpg hwy- with cruise control.... biggest reason I want to do the intake, and exhaust job, and do a flash tune. (People that have just done the tune, say they got 3-4mpg better- also interchangeable settings, economy, sport, tow, etc. and you input what size tires you have- tuner is for jeeps obviously)
 
#26 ·
the stock skids are removed and replaced with aftermarket skids = tummy tuck.. that is how you do it on a JK.... River raider is one company

I quote "There is almost 1.75" of ground clearance gained by ditching the stock skids! This is the same as going from a 35" to 38" tire. In addition to smoothing out the belly you can see why this is a smart investment."
JKOwners.com - The community for Jeep JK Owners

use this gear ratio calc. it is one of the best.... plug in transmision and transfer case and you can see alot of detail calculation and compare from stock to your target ratio or compare 2 gear ratios.... I have someone who has 5:13's with 35's. he is keeping that and going to 37"s soon. he thinks its enough gear. had for me to say... I am likely going 5:13's base on this calculator
http://www.grimmjeeper.com/metric_gears.html
 
#28 ·
All I know is that this thread has me so motivated to get a jku ASAP! I love the idea of being able to run even 35" tires on a 2.5" lift let alone 37"s I think I'm going to start purchasing all the mods I want for it now so once I get stuck with that payment every month I'll already have everything I want done haha
 
#29 ·
Not to rain on your parade bro, but if your monthly payment has you worried, $10k of mods doesn't seem in the budget. (in the immediate future) I make $700/day, ($3,200/wk after taxes) and spending $5k on a vehicle that's already paid off, is a big spending for me..... I'd get the vehicle first, and then make it your pet project over the next 5-6 years, of slowly adding mods to it. If you buy all the mods first, one.. it's NEVER enough, and two, you'll have $10k of parts in your garage, pantry, and bedroom closet, and still be driving an '83 honda civic. Also, if you do it that way, you'll appreciate it more. Buy a jeep and immediately deck it out, and it's old in a few months... put a new part on it every few months, and you appreciate it (and every individual upgrade) more and more. Don't destroy the moment by getting everything right away. More or less, make it a way of living, not a single moment of joy.

Fricken elegant huh?
 
#30 ·
That's depressing if it already has 5.38s.... guess air intake, exhaust, and tune, are the only way I'm going to see any umph out of it. Tune probably the most. I feel like it has power, but it just gets confused what gear it should be doing it in. Maybe it has 5.13s.... It's depressing to think that it already has what I want. Cause it's not that spectacular.

Still though, that conversion thing looks like a hieroglyph diagram to me. Don't know what you expect me to do with it. lol
 
#31 ·
wendelb,
I have 37s and 5.38s, and at 75 mph I'm turning 2600 RPM. You can't have 5.38s if you don't have a Rubicon front axle. That, combined with knowing that for each step up in gear ratio you gain about 100 RPM at highway speed, makes me think you have 5.13s. If you're at 2500 RPM at 75 mph that is exactly what I would expect for 37s and 5.13 gears.

I had 5.38 gears with 35" tires on a previous Jeep, and it was perfect, so I wouldn't be surprised if the previous owner went with 5.13s on your Jeep.

I get 14 mpg with my JK, not much better than yours.

One way to determine your gear ratio is to tape a string to the side of your rear driveshaft, with a white chalk mark at the start of the string for an index mark. Jack up the rear axle and rotate the rear tires exactly one full turn. If you have 4.88 gears the string will be wrapped around your driveshaft 4.9 times. If you have 5.13 gears the string will be wrapped around your driveshaft 5.1 times. Now, the tough part is going to be determining if it is 4.9 wraps, or 5.1 wraps, because that isn't much difference.
 
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