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Old 09-03-2012, 11:55 AM   #1
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Gearing kills gas mileage?

Everyone I talked to around here about gearing my 2003 automatic TJ from 3.07's to 4.10's have told me it will kill gas mileage. I'm wanting to go to 33's and I think the 4.10's would be good for those. I thought gearing for bigger tires brings it back to stock like mileage and ride?

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Old 09-03-2012, 11:58 AM   #2
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Not true. It will help your power and gas Milage, as well as reduce the stress on your transmission. You will either want 4.10 or 4.56 years, I don't drive an automatic so im not sure what's best for you. Im sure others who know more about gearing for an automatic will chime in shortly

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Old 09-03-2012, 12:06 PM   #3
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Thats what I thought. Thanks!
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Old 09-03-2012, 12:32 PM   #4
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The 3 spd auto would need 3.73 and the 4 spd would need 4.10's I believe? That's my guess LOL. I know the 5 spd needs 4.56
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Old 09-03-2012, 12:36 PM   #5
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Originally Posted by wethington2011 View Post
Everyone I talked to around here about gearing my 2003 automatic TJ from 3.07's to 4.10's have told me it will kill gas mileage. I'm wanting to go to 33's and I think the 4.10's would be good for those. I thought gearing for bigger tires brings it back to stock like mileage and ride?
the 03-06 42RLE auto needs 4.88 gears for 33s. dont forget to recalibrate the speedo by installing a new speedo gear.

btw, if you have an 03 42RLE non-Rubi, you won't have 3.07s. you should have 3.73s.
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Old 09-03-2012, 12:42 PM   #6
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Yeah I have the 03 42RLE, did not know that it had 3.73's in it. Even with the X model? From what I hear it is the lowest model you can get
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Old 09-03-2012, 12:43 PM   #7
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Yeah I have the 03 42RLE, did not know that it had 3.73's in it. Even with the X model? From what I hear it is the lowest model you can get
go look at the tag on the diff and you tell us. you have the Jeep.

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Old 09-03-2012, 12:45 PM   #8
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Already tried that, to much corrosion to even begin to read it. Might take a wire brush to it, it could help
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Old 09-03-2012, 04:00 PM   #9
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You could also lift the front or rear end off the ground (use jack stands) and rotate the tires 2 complete revolutions and count the revolutions of the appropriate drive shaft. Or, take the cover off one of the diffs and count the teeth on the pinion and ring gears. Divide the no of teeth on the ring gear by the teeth on the pinion to get your ratio.
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Old 09-03-2012, 06:34 PM   #10
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I wiped the crap off and I do indeed have 3.73's which makes me pretty happy. So the next question. I have now seen 4.10, 4.56, and 4.88. Which one would be best with 33X12.50 15?
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Old 09-03-2012, 06:53 PM   #11
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4.10 for highway cruising..4.56 for around town..4.88 for pulling
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Old 09-03-2012, 06:58 PM   #12
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4.10 for highway cruising..4.56 for around town..4.88 for pulling
4.10 is what most prefer.
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Old 09-03-2012, 07:02 PM   #13
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4.10 is what most prefer.
I agree
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Old 09-03-2012, 07:03 PM   #14
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Originally Posted by wethington2011 View Post
I wiped the crap off and I do indeed have 3.73's which makes me pretty happy. So the next question. I have now seen 4.10, 4.56, and 4.88. Which one would be best with 33X12.50 15?
with the 42RLE, you want 4.88s for 33s. 5.13s or 5.38s with 35s.

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4.10 for highway cruising..4.56 for around town..4.88 for pulling
this comment has no regard to the transmission involved.

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4.10 is what most prefer.
not with the 42RLE.
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Old 09-03-2012, 07:04 PM   #15
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Quote:
Originally Posted by UnlimitedLJ04

with the 42RLE, you want 4.88s for 33s. 5.13s or 5.38s with 35s.

this comment has no regard to the transmission involved.

not with the 42RLE.
I thought he had a 3 spd I'll shut up now
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Old 09-03-2012, 07:07 PM   #16
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Quote:
Originally Posted by wethington2011 View Post
Everyone I talked to around here about gearing my 2003 automatic TJ from 3.07's to 4.10's have told me it will kill gas mileage. I'm wanting to go to 33's and I think the 4.10's would be good for those. I thought gearing for bigger tires brings it back to stock like mileage and ride?
Your automatic is the 4-speed and it needs a much (!) lower ratio for 33" tires due to its steep .69 (IIRC) Overdrive ratio. The optimal ratio for your particular TJ and your particular 4-speed 42RLE transmission with 33" tires is 4.88. Yes, 4.88. 4.56 would be ideal for the 5-speed manual and 4.10 would be ideal for the 32RH 3-speed automatic... but your 4-speed 42RLE really needs 4.88 to be geared optimally.

After my previous TJ was stolen, I bought a used '04 with the same transmission you have and 35" tires. The previous owner installed 4.88 and he has even admitted it was a mistake. For 35" tires, 4.88 is not eough gear as it causes the engine to lug on the highway... the same thing it would do with 4.56 and 33" tires. For my TJ that has the same transmission and engine yours has, 5.13 or even 5.38 would be optimal for 35" tires. For your TJ 4.88 would be optimal for 33" tires.

Don't even consider 4.10 or 4.56, that would only be the right choice for other transmission types.

And gearing only kills mileage when it isn't optimal for the tire size. In the case of 4.88 for your 33" tires, that is the optimal ratio so it would also allow your engine to work in its proper power band which means the best mpg.
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Old 09-03-2012, 08:49 PM   #17
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4.88 it is! Thanks for the help, I was going to go with 4.10's without even asking. I am glad I did now
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Old 09-03-2012, 11:57 PM   #18
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Just a note - If you have a Dana 30 in the front, which you probably do with a Dana 35 rear in a non-Rubicon model, the lowest gears available are 4.88. I have a 4cyl 5 speed 97 TJ on 35" tires. It had 4.10 gears when I bought it. It drove ok, but 5th gear was pretty useless. I installed 4.88 gears when I switched the rear axle to an 8.8. It's much better, but should still be in the 5.something range. It's just too expensive to upgrade the complete front axle!

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