We installed 4.88s from a 4.10. (37” tire). Review, tips, and notes inside!

[JT1]

Often when discussing with someone, they fail to explain themself and things stay vague.

Thankfully, that's not a problem. You did a great job explaining your position.

You are still mistaken though. Where the torque is multiplied makes no difference to the engine.

If your transmission is reducing 3 to 1 and your differential is reducing 4 to 1

ABSOLUTELY IDENTICAL

to the engine as if the transmission was reducing 4:1 and the diff 3:1.

The overall reduction is 12:1. Period.

Either of these examples will make the same power, generate the same heat, and use the same amount of fuel.

As far as your statement that "motor speed is now too high and over heats". That's also incorrect because the heat generated by a motor is directly proportional to how much power it is being asked to produced.

Power = torque x RPM.

In other words, a motor putting out 100 ft lbs of torque at 3000 rpm is making the same power as a motor putting out 300 ft lbs of torque at 1000 rpm. Both will make similar heat.

What you are saying here is true. In my case with a manual, 3.73s with 37's, 4th gear is nearly identical to 6th with 5.13s. Talking 20rpm different at the same road speed, the difference comes in when you need to downshift. from 6th to 5th is 300 rpm with 5.13s. With 3.73s 4th to 3rd is a 1300rpm jump. The gear spread with the 5.13s is tighter, and you can drive it like a semi and not bog. Anything under 1800 rpm and the pentastar falls on its face.

The 8spd is a different animal.

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[dcmdon]

What you are saying here is true. In my case with a manual, 3.73s with 37's, 4th gear is nearly identical to 6th with 5.13s. Talking 20rpm different at the same road speed, the difference comes in when you need to downshift. from 6th to 5th is 300 rpm with 5.13s. With 3.73s 4th to 3rd is a 1300rpm jump. The gear spread with the 5.13s is tighter, and you can drive it like a semi and not bog. Anything under 1800 rpm and the pentastar falls on its face.

The 8spd is a different animal.

This is a fantastic point.

Lower gears in transmissions have a larger spread between gears. So with a lower axle ratio, you are in higher gears and are therefore working with gears with a lower spread more of the time.

A lower rear end is also more important with a manual because it reduces clutch abuse when you need to get rolling in a difficult situation.

Back in the early 90s I had a Subaru wagon with all of 80 hp. But it had a low range for its 4wd system. So if I was pulling my boat out on a ramp, I could get the clutch out really fast and minimize clutch abuse. Ha.

 

[Wheelin98TJ]

Absolutely. It will launch harder. It will therefore be faster almost all the time.

But sometimes you get bit by this logic. Back in 2004 Subaru came out with the STi to compete against the Mitsubishi Evo. The Evo was quicker to 60. But that didn't really tell the story.

Up until 57 mph, the Subaru was actually faster. But because the Evo had just a bit taller gearing it could get to 60 in 2nd gear.

The Subaru bounced off its rev limiter at 58 mph and required a shift to 3rd JUST before 60.

So to get to 60 the higher geared Evo required 1 shift. The lower geared Subaru required 2.

The Evo required a shift into 3rd at 64 mph. At which point the Subaru retook the lead.

So you see, sometimes lower gearing bites you in the butt. And sometimes it helps. It all depends on what you are looking for.

Another example. I used to road race a 125 and 250 cc 2 stroke GP bike. We would usually gear the bike so it hit redline in top gear at the fastest point on the track. But sometimes gearing it like that would put you in a place where you had to upshift immediately before an important corner,. Shifting into that gear, to only hold it for a split second was slower than if you went down a tooth on the rear sprocket (gearing up) so that you could hold the lower gear into the corner.

I hope this makes sense.

I understand what you're saying here.

It was your original statement "So once you are rolling over 10 mph, and the transmission is in 1st through 7th, the axle ratio is irrelevant." that I did not understand.

What you are explaining above sounds like the axle ratio is pretty relevant.

 

[JT1]

This is a fantastic point.

Lower gears in transmissions have a larger spread between gears. So with a lower axle ratio, you are in higher gears and are therefore working with gears with a lower spread more of the time.

A lower rear end is also more important with a manual because it reduces clutch abuse when you need to get rolling in a difficult situation.

Back in the early 90s I had a Subaru wagon with all of 80 hp. But it had a low range for its 4wd system. So if I was pulling my boat out on a ramp, I could get the clutch out really fast and minimize clutch abuse. Ha.

100% this. I can take off in 1st with no gas pedal now. Or take off up an incline with no drama.

 

[Bonanza]

Filled up again- 16.3 MPG.

 

[NachoRuby]

Yes, but if you need an overall lower ratio, you just go down a gear.

At some point 7th with an X rear end is the same as 6th with a Y rear end. Same overall reduction ratio.

You can't really downshift to first easily once moving though, without forcing it once you hit about 8 mph or so. And on 35s, second doesn't accelerate very well until about 16-17 mph. So it absolutely would make a difference.

It would allow you to use second and third in traffic around town, without forcing the shift to first once you get rolling, once you slow down in 10 feet when traffic starts crawling.

And it would shorten the ridiculously long 3rd gear, which ends at over 104 mph on 34.4" tires (if Rubicons/Mojaves weren't limited to 97, and you had a death wish). At 4.88, it would make that a more reasonable and usable 88mph, which means you could be in the meat of the powerband once you downshift to 3rd or 4th for merging or passing. It will make all the gears more usable.

Remember, the manuals only have 6 to choose from, and the ratios aren't close, so 3rd on 35s accelerates too slowly for merging or passing, especially while climbing a steep hill, and you're in the wrong part of the powerband. But second would be a money shift (and also would be ridiculous at highway speeds). Regearing would tame 3rd and 4th a bit, making the Jeep more drivable. With stock gearing on 35s, you have to shift to 4th to pass on the interstate, or possibly third if you need to haul butt, or climb a mountain at the same time. But third is still too long.

I'm not going to regear, because frankly, it's just too expensive for me, and I don't want the warranty issues, but it would make a tremendous difference. 4.1s are fine on 33s, and OK on 35s. It's doable. It's usable. But 4.88 or 5.13 would be ideal for the most drivability once you up your tires on the manual to 35s. If you're going to 37s or more, it's essential.

 

[JT1]

You can't really downshift to first easily once moving though, without forcing it once you hit about 8 mph or so. And on 35s, second doesn't accelerate very well until about 16-17 mph. So it absolutely would make a difference.

It would allow you to use second and third in traffic around town, without forcing the shift to first once you get rolling, once you slow down in 10 feet when traffic starts crawling.

And it would shorten the ridiculously long 3rd gear, which ends at over 104 mph on 34.4" tires (if Rubicons/Mojaves weren't limited to 97, and you had a death wish). At 4.88, it would make that a more reasonable and usable 88mph, which means you could be in the meat of the powerband once you downshift to 3rd or 4th for merging or passing. It will make all the gears more usable.

Remember, the manuals only have 6 to choose from, and the ratios aren't close, so 3rd on 35s accelerates too slowly for merging or passing, especially while climbing a steep hill, and you're in the wrong part of the powerband. But second would be a money shift (and also would be ridiculous at highway speeds). Regearing would tame 3rd and 4th a bit, making the Jeep more drivable. With stock gearing on 35s, you have to shift to 4th to pass on the interstate, or possibly third if you need to haul butt, or climb a mountain at the same time. But third is still too long.

I'm not going to regear, because frankly, it's just too expensive for me, and I don't want the warranty issues, but it would make a tremendous difference. 4.1s are fine on 33s, and OK on 35s. It's doable. It's usable. But 4.88 or 5.13 would be ideal for the most drivability once you up your tires on the manual to 35s. If you're going to 37s or more, it's essential.

I really want to try a manual on 35s with 4.56s..
Now that you can order a manual rubi wrangler with factory 4.88s for $995, you would think that option or other ratios gets carried over to the gladiator too.

 

[vst]

Hey guys I just bought a Jeep gladiator rubicon it has 37‘s on it already Yokohama x-at’s I was wondering if you could tell if it’s re geared and maybe what ratio. I know the only way is to open up the diff and check but on the highway in 8th gear at 70mph I’m at 2200 rpm’s. I think if it had the stock 4.10’s my rpm’s would be lower than that but not sure this is my fist Jeep. Any info would be appreciated thanks

 

[Bonanza]

Hey guys I just bought a Jeep gladiator rubicon it has 37‘s on it already Yokohama x-at’s I was wondering if you could tell if it’s re geared and maybe what ratio. I know the only way is to open up the diff and check but on the highway in 8th gear at 70mph I’m at 2200 rpm’s. I think if it had the stock 4.10’s my rpm’s would be lower than that but not sure this is my fist Jeep. Any info would be appreciated thanks

You can download Jscan app on your phone, with an OBD2 widget and check the programming. It would be a bit of a challenge to visually guess the gear by opening the diff cover, but I suppose you could do it if you were so inclined. The best way is checking to see if the computer was flashed to a new gear size.

 

[OngsterA]

This is a fantastic point.

Lower gears in transmissions have a larger spread between gears. So with a lower axle ratio, you are in higher gears and are therefore working with gears with a lower spread more of the time.

A lower rear end is also more important with a manual because it reduces clutch abuse when you need to get rolling in a difficult situation.

Back in the early 90s I had a Subaru wagon with all of 80 hp. But it had a low range for its 4wd system. So if I was pulling my boat out on a ramp, I could get the clutch out really fast and minimize clutch abuse. Ha.

My need for going 5.13s (or 4.88) later is once I go up to 37s. Right now I've crawled steep sand and rocks on 35 KM3s and the stock 4.10s are just fine. It's the clearance w 37s that will help a bit more. When these wear out, 37s will go on. I will appreciate the 1st gear (ZF 8 spd) higher torque on these situations, easier load on the trans and rear. I know mpg will suffer but that's my main goal for use.

 

[Wheelin98TJ]

Hey guys I just bought a Jeep gladiator rubicon it has 37‘s on it already Yokohama x-at’s I was wondering if you could tell if it’s re geared and maybe what ratio. I know the only way is to open up the diff and check but on the highway in 8th gear at 70mph I’m at 2200 rpm’s. I think if it had the stock 4.10’s my rpm’s would be lower than that but not sure this is my fist Jeep. Any info would be appreciated thanks

RPM x tire diameter
---------------------------- = Gear ratio
Trans ratio x MPH x 336


2,200 x 37
-----------------
0.67 x 70 x 336


81,400
---------- = 5.165
15,758.40


It appears you have 5.13 gears.

 

[JT1]

Hey guys I just bought a Jeep gladiator rubicon it has 37‘s on it already Yokohama x-at’s I was wondering if you could tell if it’s re geared and maybe what ratio. I know the only way is to open up the diff and check but on the highway in 8th gear at 70mph I’m at 2200 rpm’s. I think if it had the stock 4.10’s my rpm’s would be lower than that but not sure this is my fist Jeep. Any info would be appreciated thanks

That looks like 5.13s...

 

[vst]

Thanks a lot 5.13’s is what I was thinking based on the research I was doing

 

[NachoRuby]

I really want to try a manual on 35s with 4.56s..
Now that you can order a manual rubi wrangler with factory 4.88s for $995, you would think that option or other ratios gets carried over to the gladiator too.

You'd think, being a truck, the gladiator would get it first.

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