I run 35s with an automatic and have the 4.88 gears. I'm happy with the setup. It gives an overall gear ratio similar to the stock setup but just slightly lower. I find the offroad behavior to be about right for me. I also use my Jeep as a daily driver and think it works pretty well for that, too. I don't have a way to compare it with a 5.13 setup but am happy with my choice.
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G
I went with 5.13s (I’m running 37s) because I regularly drive up some big mountain passes in Colorado. The reality is that this thing is not great on big hills stock or with the 5.13s. I’ve just gotten used to it, take my time when I’m driving and take my other car when I’m not wheeling 
OK - some perspective may help here.
Going from the stock 32's, to 35's, is going to drop your RPM's by ~ 8.6% in every gear.
So, what ever rpm you are at with the 32's and 4.10's, will be ~ 8.6% lower with 35's and 4.10's.
Swapping in 4.88's RAISES your rpm for any given speed by ~ 16%
So, therefore, the OVERALL CHANGE in going from 32's and 4.1's, to 35's and 4.88's, is ~ a 7.7% RISE in RPM for any given speed.
So - If you are going along at 2,000 rpm with 32's and 4.10's...and kept going the same speed with 35's and 4.88's...your RPM would RISE to ~ 2,148 RPM.
This is pretty damn close...a few more rpm to help with leverage of the larger tires...and roughly stock acceleration, etc.
Of course, stock acceleration doesn't please every one, and, to sacrifice mpg for more oomph, you could gear deeper, to the 5.13's, etc.
The 5.13's RAISE the rpm's in every gear ~ another 4.9% over that of the 4.88's...
So...that 2,000 RPM that went to 2,148 RPM...now goes a bit higher, to ~ 2,253 RPM.
Hardly night and day, but ~ 250 RPM over the stock set-up none-the-less.
To take it in the opposite direction...
Lets say the 4.10's are cruising along at 2,000 RPM with the 32's, and you swap on 35's, WITHOUT re-gearing...
OK, your RPM DROP by ~ 8.6%, to ~ 1,828 RPM, or by ~ 172 RPM.
If that 172 rpm drop doesn't phase you, well, you could even keep the 4.10's in the diffs, pocket the extra MPG, and drive happy.
(You will have essentially given yourself the STOCK cruising RPM for the higher MPG base model JK...as that's the RPM range your fuel economy is peaked at.)
Hope that helps a bit!
Going from the stock 32's, to 35's, is going to drop your RPM's by ~ 8.6% in every gear.
So, what ever rpm you are at with the 32's and 4.10's, will be ~ 8.6% lower with 35's and 4.10's.
Swapping in 4.88's RAISES your rpm for any given speed by ~ 16%
So, therefore, the OVERALL CHANGE in going from 32's and 4.1's, to 35's and 4.88's, is ~ a 7.7% RISE in RPM for any given speed.
So - If you are going along at 2,000 rpm with 32's and 4.10's...and kept going the same speed with 35's and 4.88's...your RPM would RISE to ~ 2,148 RPM.
This is pretty damn close...a few more rpm to help with leverage of the larger tires...and roughly stock acceleration, etc.
Of course, stock acceleration doesn't please every one, and, to sacrifice mpg for more oomph, you could gear deeper, to the 5.13's, etc.
The 5.13's RAISE the rpm's in every gear ~ another 4.9% over that of the 4.88's...
So...that 2,000 RPM that went to 2,148 RPM...now goes a bit higher, to ~ 2,253 RPM.
Hardly night and day, but ~ 250 RPM over the stock set-up none-the-less.
To take it in the opposite direction...
Lets say the 4.10's are cruising along at 2,000 RPM with the 32's, and you swap on 35's, WITHOUT re-gearing...
OK, your RPM DROP by ~ 8.6%, to ~ 1,828 RPM, or by ~ 172 RPM.
If that 172 rpm drop doesn't phase you, well, you could even keep the 4.10's in the diffs, pocket the extra MPG, and drive happy.
(You will have essentially given yourself the STOCK cruising RPM for the higher MPG base model JK...as that's the RPM range your fuel economy is peaked at.)
Hope that helps a bit!
I was getting 14mpg with 4.10's and 16.2 with 5.13's. I drive all over the place (Big bear, San Diego, Imperial, etc) and mine will run better given a little extra RPM. I also did this knowing all the weight I would be adding (bumpers, winch, tires, etc) the extra gear wouldn't hurt me.
Plus I can step up to 37's if I want to down the road...
75 mph is 2500 rpm with 35's and 5.13's in my Rubi.
And I have the chip/speed correction/gears handled, Teej...
Good breakdown Teej...
Riddle me this:
32's and 4.1's, With all the heavy poly skids, Shrockworks bumpers and winch, I am getting 18mpg @ 65mph.
Would 35's and 4.88's yield the same MPG at the same speed?
What about the strain on the transmission from trying to compensate for the massive increase in rolling mass?
I am looking for the best of both worlds gearing. I would like 35's, but i am worried my MPG will realy drop.
hehe... This topic will never have a definite one size fits all solution!
Riddle me this:
32's and 4.1's, With all the heavy poly skids, Shrockworks bumpers and winch, I am getting 18mpg @ 65mph.
Would 35's and 4.88's yield the same MPG at the same speed?
What about the strain on the transmission from trying to compensate for the massive increase in rolling mass?
I am looking for the best of both worlds gearing. I would like 35's, but i am worried my MPG will realy drop.
hehe... This topic will never have a definite one size fits all solution!
Tires and MPG can be tricky...for example, going from 32's to 35's, not much real life change...but, going a bit more to 37's...big difference, as the hysteresis and rolling resistance issues at that break point really whack into you significantly.
The larger tires act as a a deeper overdrive, which can help real world numbers.
What happens to a lot of people though...
They speed, which eats gas (Their speedometers are off, so they drive at what the speedometer SAYS is a good speed, but its LYING to them, they are REALLY going faster than it says)
They are also feeling like the truck is slower to get upto speed, because it SAYS it is....accelerating to 70 mph DOES take longer, if you use the 70 on the speedometer.
The MPG METER on the DASH is lying too...as you went MORE MILES than it thinks you went (Its also lying to you...).
In short, when you get 35's, you drive faster, try to accelerate to higher speeds, have to down shift more to maintain the higher speeds, go further than the odometer says, and get better mpg than the mpg dash gauge indicates, etc.
That's typical anyway.
As the higher gears (4.88's, 5.13's, etc...) change what the speedometer and odometer read, as speed is read from the tail shaft output, your mpg will appear to go up again after re-gearing...because its the same as installing SMALLER tires.
IE: You are now NOT speeding, you are going SLOWE than before, you are NOT accelerating up to the same speed, you are not having to downshift as often to maintain the higher speed (Because you now use a lower real speed...), and, you are going FEWER miles than the odometer says, and, you are getting WORSE mpg than the mpg gauge sys, etc...)
People who get the chip/correction of speed and gearing, etc....get a true picture of their mpg, etc...as the electronic idiocy is reined in.
The larger tires act as a a deeper overdrive, which can help real world numbers.
What happens to a lot of people though...
They speed, which eats gas (Their speedometers are off, so they drive at what the speedometer SAYS is a good speed, but its LYING to them, they are REALLY going faster than it says)
They are also feeling like the truck is slower to get upto speed, because it SAYS it is....accelerating to 70 mph DOES take longer, if you use the 70 on the speedometer.
The MPG METER on the DASH is lying too...as you went MORE MILES than it thinks you went (Its also lying to you...).
In short, when you get 35's, you drive faster, try to accelerate to higher speeds, have to down shift more to maintain the higher speeds, go further than the odometer says, and get better mpg than the mpg dash gauge indicates, etc.
That's typical anyway.
As the higher gears (4.88's, 5.13's, etc...) change what the speedometer and odometer read, as speed is read from the tail shaft output, your mpg will appear to go up again after re-gearing...because its the same as installing SMALLER tires.
IE: You are now NOT speeding, you are going SLOWE than before, you are NOT accelerating up to the same speed, you are not having to downshift as often to maintain the higher speed (Because you now use a lower real speed...), and, you are going FEWER miles than the odometer says, and, you are getting WORSE mpg than the mpg gauge sys, etc...)
People who get the chip/correction of speed and gearing, etc....get a true picture of their mpg, etc...as the electronic idiocy is reined in.
I am 99% sure that is not the case on the JK, it gets it's speed from the 4 wheel sensors. Changing gears in the JK does not effect the speedo.
I heard different.
The engineers at Chrysler say output...not the ABS sensors, etc.
Of course, they COULD be wrong....but it would not explain people's speedo's suddenly changing the reading relative to the new diff gears and their GPS, etc....might be a coincidence, etc....plenty to go around.
The engineers at Chrysler say output...not the ABS sensors, etc.
Of course, they COULD be wrong....but it would not explain people's speedo's suddenly changing the reading relative to the new diff gears and their GPS, etc....might be a coincidence, etc....plenty to go around.
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