Originally Posted by LidLess07X

My new JK is an Unlimited Sport and came with the 3:73s and an automatic. I slapped on a set of 35s and thought I was okay at first but later found my transmission searching back and forth between 3rd and 4th at highway speeds. Just wanted to say for anyone unsure about 4:88 vs 5:13 that I am completely satisfied

Originally Posted by jknit

any increases in gas mileage?

Originally Posted by planman

I have no doubt that LidLess07X is happy with his 4.88s--because he doesn't have 5.13s or 5.38s to compare to the 4.88s.

In case someone wants more than an anecdotal explanation of gearing and rpm levels:


The JK engine needs around 2500 rpm to produce sufficient rear wheel horsepower to operate efficiently.

Here is the torque and horsepower curve comparing stock to a Superchips 91 octane tune:




Notice how steep the lower, rear wheel horsepower line in the graph is. Stock rear wheel horsepower at 2000 rpm is only about 60 compared to the Superchips tuned rear wheel horsepower at around 2800 rpm is over 100, and at 3000 rpm it is about 110.

Stock torque at 2000 rpm is about 150 and the tuned torque at about 2800 rpm is about 190. At 3000 rpm it is over 190.

Those increases from 60 rear wheel horsepower to 100+ or 110+ rear wheel horsepower and 150 torque to 190+ torque are quite meaningful.

So, if you are concerned about the power capability of the JK engine, the first thing you want to do is make sure your wheel/tire/gear combination puts you at over 2500 rpm at highway speeds.

Next is a chart of rpm levels by gearing and transmission type at 70 mph:




You would need 5.13 or 5.38 gears with stock size Rubicon 32" tires to put you at 2600-2700 rpm with an automatic. Essentially, this means that any larger tire on an automatic will put you at the lower end of the horsepower and torque numbers--even with a regear.

However, a stock 6 speed Rubi with stock 32" tires in 6th gear puts you over 2500 rpm at 70 mph. 4.88s and 35" tires (that actually measure 34" mounted) puts you at around 2800 rpm at 70 mph.

So, due to transmission gearing, a 6 speed with 4.88s and 35" tires you can run in a better power range than stock.

With larger tires, you need better than stock power because you are turning heavier, larger tires.

Also, lighter tires will help with performance. For example, load range C 35x12.50R15 tires on aluminum 15x8 wheels are significantly lighter than load range E 35x12.50R17 tires on 17x8 steel wheels.



With 5.13 gears, a 6 speed, and 37" tires (actual mounted height around 36"), my 2 door JK Rubi would run around 2800 rpm at 70 mph. I could drive most any freeway grade at 3000 rpm with my Superchips tune because that put me in the 110 rear wheel horsepower and 190+ torque range.

Whereas a stock automatic JK has no chance of driving a steep freeway grade without gear hunting/constant downshifting because they are running less than 2000 rpm and less than 60 rear wheel horsepower. Then, add heavy, larger tires/wheels to the equation with no tune or regear, and the people who you may have spoken with that suggest that you feel you can almost walk faster are right.


LidLess07X's choice of 4.88s on 35s (that maybe measure 34" mounted) puts him at 2300-2400 rpm at 70 mph, or about 70 rear wheel horsepower and maybe 160 torque.

He is happy with his increase from his prior 50 rear wheel horsepower increase to 70 and 150 rear wheel torque to 160.

Had he regeared to the almost universally recommended 5.13s with an automatic non-Rubi and added a Superchips 91 octane tune he'd be at 90 rear wheel horsepower instead of 70 and over 185 rear wheel torque instead of 160.

The combination of the 5.13s and Superchips would dramatically improve his overall performance and fuel economy due to reduced tranny downshifting.


Anecdotally, he is happy with his choice.

Logically, he will have lower fuel economy and poorer performance compared to 5.13s. Financially, the 4.88s are a more expensive choice over time for poorer performance.