4.88 Gears Not What I Expected

M

Mr.Mango

New member
Not sure if I am just stupid...
I recently got my 2010 JKRU 6speed re-geared to 4.88.
At first it definitely "seemed" peppier, going up hills I didn't need to drop down to 4th to get up etc, but now I am wondering if I am just "hoping" it's better.
Steep hills, 5-6% grades on the hiways, I still drop down to 40mph in 6th and unless I gear down, I just keep getting slower.
The ratio "chart" says at 70mph I should be around the 2700rpm mark but I am closer to 2900rpm which is 5.13 gears, so I am not inclined to believe that.
I elected to be zipping around like other vehicles. A huge difference.
Am I expecting too much from this gearing change? I had a difficult time with the shop who did the work but I don't believe they would do something like this (leave the 4.10s in). I will crack the diff later this summer and count the teeth on the gears but if anyone else went from 4.10 to 4.88 with the 3.8 6speed could they share their experiences, maybe I am just being paranoid.
Should add I am running 35s.
Thanks all.
 
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I couldn't imagine going from 4.10-4.88 would make that much of difference, 3.21-4.88 on the other hand I could see making a larger difference. I have 3.21s that's why I say that but I'm a rookie so I don't really know, if you're running 35s I heard 5.13 are preferable with the 3.8 because of its lack of power
 
You might already know this, but you don't have to count the gears to find out what you have.

Jack up the rear so the tires are off the ground, make a mark (I use a piece of tape) on the tire at 12 o'clock and make a mark on the drive shaft you can see.
Rotate the tire one revolution and count how many times the drive shaft rotates. If it turns just a little over 4 full revolutions, you have 4:10. If it turns 4 and a half times = 4:56.
Almost 5 full = 4:88 and so on.

.
 
kaptkrappy said:
You might already know this, but you don't have to count the gears to find out what you have.

Jack up the rear so the tires are off the ground, make a mark (I use a piece of tape) on the tire at 12 o'clock and make a mark on the drive shaft you can see.
Rotate the tire one revolution and count how many times the drive shaft rotates. If it turns just a little over 4 full revolutions, you have 4:10. If it turns 4 and a half times = 4:56.
Almost 5 full = 4:88 and so on.

.
Click to expand...

Will try this later today and report back.
Thanks!
 
So I put the Jeep on jack stands, marked a spot on the drive shaft and tire and then rotated the tire. Only one tire rotated, so Googled this test and saw that if only one tire spins, rotate it twice.
Doing this I got 4 full spins and it was just shy of coming round for the fifth. So it looks like, to me anyways, that they are 4.88.
Happy to confirm that, just not happy with the gears. Ultimately this is a 3.8 and maybe with the 3.6 it would be a different result overall. Maybe it is just me with high expectations.
 
Mr.Mango said:
So I put the Jeep on jack stands, marked a spot on the drive shaft and tire and then rotated the tire. Only one tire rotated, so Googled this test and saw that if only one tire spins, rotate it twice.
Doing this I got 4 full spins and it was just shy of coming round for the fifth. So it looks like, to me anyways, that they are 4.88.
Happy to confirm that, just not happy with the gears. Ultimately this is a 3.8 and maybe with the 3.6 it would be a different result overall. Maybe it is just me with high expectations.
Click to expand...

The other thing to remember is that 6th gear is an overdrive gear. It's simply there for fuel economy.
 
thardy said:
The other thing to remember is that 6th gear is an overdrive gear. It's simply there for fuel economy.
Click to expand...

Never thought about that. Course, you don't really think about fuel economy and a JK, so I will cut myself some slack on this.
 
Mr.Mango said:
Not sure if I am just stupid...
I recently got my 2010 JKRU 6speed re-geared to 4.88.
At first it definitely "seemed" peppier, going up hills I didn't need to drop down to 4th to get up etc, but now I am wondering if I am just "hoping" it's better.
Steep hills, 5-6% grades on the hiways, I still drop down to 40mph in 6th and unless I gear down, I just keep getting slower.
The ratio "chart" says at 70mph I should be around the 2700rpm mark but I am closer to 2900rpm which is 5.13 gears, so I am not inclined to believe that.
I elected to be zipping around like other vehicles. A huge difference.
Am I expecting too much from this gearing change? I had a difficult time with the shop who did the work but I don't believe they would do something like this (leave the 4.10s in). I will crack the diff later this summer and count the teeth on the gears but if anyone else went from 4.10 to 4.88 with the 3.8 6speed could they share their experiences, maybe I am just being paranoid.
Should add I am running 35s.
Thanks all.
Click to expand...

Note that the chart is set for actual tire size. I would guess that your 35" tires are more like 34". On my 3.6 I went from 4.10 to 4.88 and noticed a big difference, but I have an auto as well.
 
I went from 3.73 to 4.88 on 08JKUX. Very happy with them. Had 35's was about 2800-2900 rpms @ 70. Now have 37s and about 2600-2700 rpms. Power issue could be a compression issue with the motor. The bottom ring is to small stock and gum up causing issues. I just did a rebuild and can climb 5% grades in 6th. Slows down to about 55mph. In 5th I can maintain the 70mph.
 
Stupid question, but did you reprogram your computer so it is showing you the correct information? Anyway, my friend (member name Meanst) re-geared his manual '07 JKUR with 35's from 4.10 to 4.88, reprogrammed with an AEV pro cal, and the difference is night and day. His only complaint was that he waited a year before doing the re-gear.
 
Sharkey said:
Stupid question, but did you reprogram your computer so it is showing you the correct information? Anyway, my friend (member name Meanst) re-geared his manual '07 JKUR with 35's from 4.10 to 4.88, reprogrammed with an AEV pro cal, and the difference is night and day. His only complaint was that he waited a year before doing the re-gear.
Click to expand...

You don't need to program with a manual. I thought you had to but you don't.
 
I bought the Jeep from the original owner. Lift and tires were all done at dealer. I believe they (dealer)set the speedo correctly. I have confirmed the speed via GPS.
I have consider getting a programmer but have been worried about messing with the factory settings and just not sure how much of a difference it might make.
 
Mr.Mango said:
I bought the Jeep from the original owner. Lift and tires were all done at dealer. I believe they (dealer)set the speedo correctly. I have confirmed the speed via GPS.
I have consider getting a programmer but have been worried about messing with the factory settings and just not sure how much of a difference it might make.
Click to expand...

The dealer can only program your speedo based on the largest factory tire size available for the JKs which is ~32". So you would need to get a programmer or a procal (or something similar) to set the actual tire size...
 
Reprogramming the gear ratio doesn't effect speedo. The tone rings are after the gears. Only tire size reprogramming effects speedo. On a manual you can reprogram the gear ratio but it doesn't really do much as the shifting is programmed by the lump between the steering wheel and the gas pedal :)
 
I just upgraded from 373 to 488. 15 jku 35s auto trans. I still down shift going up small hills and even in a head wind. The difference in pwr is great. But the pwr loss on hills is expected. I have a v6 not a v8
 
I have a 2015 jku rubicon. I'm running trail grapplers 35s. I'm re gearing in a couple weeks. Is 5.13 too much for 35s? I live above 5000 ft and do all my wheeling in the mountains.
 
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