Wgrin

I'd question where you are looking for gears, you should have the same price on almost any gear set whether 4.1,4.56, 4.88, or 5.13. I can't believe they wanted me to get 5.13, I'm actually going to see if they will let me swap them out, I'm sure they will say no but it won't hurt to ask.

dmhines

I geared and put 4.88s in specially for improved performace off-road. If I didn't care about wheeling and just wanted big tires I'd go with 4.10's. I guess I'm spoiled because I have motorcycles and a 2012 Mustang GT for when I want to go fast. I never rev the Jeep over 3K RPM and rarely go over 65MPH. I drove to Florida in Jeep with cruise control on 63 MPH the entire way!

DesolationMania

Here's my dilemma...yes it is a DD, but it's a DD that's going to be on Signal Peak, Hell Hole, Rubicon this summer. To be honest, I think either way I'll be happy. I'd be even happier if I could do the gearing on my own, but I read a flipping fantastic thread on doing this, and when I got done with Step 66, i knew this is just too much for me. Not to mention Ive read a few threads where "experts" didn't get it right.

Topkicker1

I'm running KM2 305/70/R17's (34x11.75) w stock 3:73 gears. It's tollerable here in Flatland, IN except when you must deal with all the wind here. My auto JK gear hunts between D and OD on the interstate more than I'd like. I will regear next year and am dead set on 4:88's with my auto and 3.8L. Most would reccomend 5:13's but like the OP, mine is a daily drive and don't want to kill the gas millage. I think I'll stick w 34's even after I regear. I think they look perfect with a 2.5 lift. They're is only 1/2in difference in diameter between the KM2 35x12.5's vs the 305's. I like the width of the 305's better. It looks cleaner IMO. I had 33x12.5's before. I didn't like the look from behind. Too much tire past the fenders for me.

jaybrown

5.13 on an auto is the same as 4.88 on a manual

Sahara Lee

Ok, let's all do the math...

Maybe I'm full of manure (and please tell me why I am) but assuming you only want to keep the same RPM at highway speed (no torque increase)...

If I go from 32" to 35" tires my TIRE revolutions per mile decrease from 649 to 598 (from tire manufacturer specs). That means to keep the same ENGINE RPMs I need to increase my gear ratio by 8.5% (649/598 = 1.085284). So:

Starting with 3.21 gears, changing to 3.73 gears over corrects by 7% [i.e.: your engine RPMs will be 7% higher... (598/649)*(3.73/3.21) = 1.07]
Starting with 3.73 gears with 35" tires you will already be way ahead of stock 32" tires with 3.21 gears.
To keep the exact same RPMs as 3.73 gears while going from 32" to 35" tires you would need 4.05 (imaginary) gears! (598/649)*(4.05/3.73) = 1.00


More mental gymnastics:
If you start with 32" tires and switch to 35" tires and...
... change from 3.21 to 4.88 gears your RPMs will increase by (598/649)*(4.88/3.21) = 1.40 That's a 40% INCREASE in engine RPMs!
... change from 3.73 to 5.13 gears your RPMs will increase by (598/649)*(5.13/3.73) = 1.27 That's a 27% increase.
... change from 3.73 to 4.56 gears your RPMs will increase by (598/649)*(4.56/3.73) = 1.13 or 13%.
... change from 3.73 to 4.1 gears your RPMs will increase by (598/649)*(4.1/3.73) = 1.01 or 1%.

These numbers agree with the "gear chart" on this site when comparing RPM values.

But RPMs don't tell the whole story. Because bigger tires have more wind resistance and rolling resistance it will take more power (gas) to move your Jeep. A higher gearing (higher RPM) will give you more torque (giddy up) for faster acceleration but you will still lose gas mileage and top end.

Just a thought. Your mileage may vary. Void where prohibited. Professional driver on a closed track. Don't try this at home.

Spyder-X

i'm running 5.13's and 37's and its perfect and my gas milage has went up by over 2mpg.

dognights

Few things to toss out when trying to figure what gears you are going to select. When looking at a sizing chart realize that is the actual tire size not the size stamped on the side of the tire. So when you are looking at a 35 on the chart you are really looking at close to what most people call a 37 tire almost.

Another thing to consider when selecting gears and why many people tend to overgear a bit is due to an increase in driveline losses. I mean on average you are adding thirty to sixty pounds per corner on your ride all of that being unsprung weight and adding to a decrease in driveline output. Many people compensate by bringing rpm up a bit to power those big tires that are robbing all the juice. Im sure everyone know this already but just tossing it out there.

I really wish there was a four speed rubicrawler secondary transfer case that had two crawl speeds and two highway speeds. To me that would be the best of both worlds, and would eliminate having to monkey with re-gearing all together for around the same cost while adding a ton of customization to the mix.

T&ERun

I had 35s and 5.13s and now I'm running 37s. I went with 5.13s because I knew I would eventually get 37-38s. I'M GLAD I DID THIS. I agree with the OP, 4.56 (or 4.88-minimal difference) if you are a daily driver and don't plan on running bigger than 35s. I personally loved the 5.13s with 35s, but I'm not daily driving it. When on the highway I would only push 70mph with the 5.13s and 35s. Awesome off road though especially with the non-rubi T-Case.

Daily driven 6 speed with 35s---4.56 or 4.88
Weekend warrier and heavy offroad with 35s---5.13

T&ERun

Correct. The way to find the "correct" diameter of the tire (when on the vehicle) is to measure from the ground to the center of the wheel and multiply by two. My 37s with 28psi measure 17.75" from the ground to the center of the wheel. That's a diameter of 35.5". This is the size that you would use to get an exact measurement. And use this size to enter in tire diameters for your Superchips Flashpaq or similar devise.

I know we're getting pretty technical here (and sorry), but just wanted to help out those that don't realize some of this. It does make a 4-5mph difference (or more) on the speedo if not done correctly.