Bingbola

I have 35's on my jk 4dr with a 3 inch lift. Would 5.13's be to much if I am not looking to go higher then 35's?

Also would 4.88's give me better milage then 5.13's?

nthinuf

Measure your tires and plug some numbers into the chart in the faq's.

My opinion: 5.13's are the gears you want. If you had a rubi, or if you plan on swapping a front 44 in, I would recommend 5.38's. Can't over-gear these auto's.
As far as mileage goes, 5.13's will keep you in OD longer. Dropping in and out is what will kill your mileage, not the extra 100 rpm.

Bingbola

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Are 5.13 going to kill my gas milage on the highway compared to 4.88's?

I have 35's, (Goodyear Duratrac 325/65/18).

Anaya

I have 35" tires and went with 5:13 performance is better than it was with stock gears and tires. MPG are little lower but better than stock gears with 35" it get around 14.5 to 16 but I live in Al so there are some hills. I think it perfect choice for 35 anything larger u better go with 5:38 if u have a rubi.

nthinuf

I take it you haven't bothered to look at the chart.

There is a whopping 100rpm difference between the two gearsets. You have an auto. With 35 or larger tires, you can NOT over-gear it.


Assuming your tires measure out to 34", you are looking at 2450 rpm @ 70mph.
To put that into perspective:
A manual rubicon driving off the assembly line is running over 2600 rpm @ 70mph.

Just my opinion, of course, but you would be better off listening to people who not only have auto JK's, but have spent the money to regear them - vs people with manuals jk's, or people that have not regeared, or are basing the decision off of their old non-jk jeep, or the car they take to the track.

But it's your money, feel free to buy into the 'screaming high rpm' theory that so many people push.

Bingbola

Ok so at the end of all of that what r u trying to say? U talked in circles

nthinuf

That you couldn't be bothered to measure your tires, open the faq's, and plug some numbers into the chart?

That you are asking because you listened to advice from people that either don't have auto JK's, or haven't regeared their auto JK's?

That a bone stock manual rubicon will have higher rpm's than you will with your new gears and 35's?

That the rpm difference between the two ratios will be negligible where mpg's are concerned?

That 5.13's will keep you in OD longer, and 'should' give better mpg's because of it?

Still talking in circles? Seemed pretty easy to understand to me...

razzer

Maybe a silly question but.......................... when i measure the tires... am i too measure it static on the jeep, on the ground? or with the wheel off the ground to get the full diameter .... you know, the squat effect looses a bit unless your running 65psi .... lol

nthinuf

On the ground with your normal psi and load. The quick and easy way is to just lay a stick or 2x4 across the top of the tire and use a tape measure to get the distance to the ground. The more accurate way is to mark the sidewall and ground, then roll forward one complete revolution, mark the ground again, measure the distance and, uhhh, whatever the mathematical formula is? Divide by pi or something?

TenForward

This will hurt your brain for a while but it should help you find the answers you're looking for.

First, tire diameter. Most tires will not measure out to what the sidewall indicates. This has a lot to do with rim width, tire pressures and on and on. What you're looking for is rolling diameter. To calculate the rolling diameter, measure the distance from the ground to the center of the wheel and multiply by 2. This is about as accurate as you can get. Now that you have the tire diameter, see the equations below.

Note: Before I lifted my Jeep: For reference RPM/Speed I have a 2009 Rubicon with the stock BFGoodrich Tires that measured at 31" in diameter. At 60 MPH, engine RPM was 1800 and at 65 MPH, it was 2000RPM. Your mileage may vary so if your rig is still stock, you might want to verify and record your actual miles vs. rpm.

Here is the math:

MPH = (RPM X Tire Diameter) ÷ (Gear Ratio X 336)
Example:
MPH = (3,000 X 26) ÷ (4.10 X 336)
MPH = 78,000 ÷ 1,377.6
MPH = 56.6


RPM = MPH X Gear Ratio X 336 ÷ Tire Diameter
Example:
RPM = 70 X 4.10 X 336 ÷ 26
RPM = 3,709


Gear Ratio = (RPM X Tire Diameter) ÷ (MPH X 336)
Example:
Gear Ratio = (3,000 X 26) ÷ (70 X 336)
Gear Ratio = 78,000 ÷ 23,520
Gear Ratio = 3.31:1


Tire Diameter =MPH X Gear Ratio X 336 ÷ RPM
Example:
Tire Diameter = 65 X 3.31 X 336 ÷ 2,700
Tire Diameter = 26.8 inches


Effective Gear Ratio = (Old Tire Diameter ÷ New Tire Diameter) X Gear Ratio
Example:
Effective Gear Ratio = (26 ÷ 28) X 3.55
Effective Gear Ratio = 3.29:1

If you want to figure this out the easy way, go to 4lo dot com/calc/gearratio.htm and plug in some numbers. If you have an auto trans, your gear ratio with o/d on is .69, t-case should be 1 to 1 or you can even calculate crawl ratios. Cool stuff!

On my rig, the calculations recommend 4.88's but these charts rarely factor in lift, load, weight, if you live at altitude or near hills. In my case, I tend to load the Jeep down with stuff I take on the trail. Also, I typically go wheeling with 4 adults in the Jeep.

I am going with 5.13's since I have an auto trans and 35" tires. I may go with a larger tire later and don't want to spend the money on gears all over again. As far as mileage goes, you have to ask yourself; did you buy the JK thinking you would get great mileage. Currently I get 14MPG with a 4" long arm lift kit and 37" (well 35") Falken Wild Peak A/T's and stock 4.10 gears. I hope to regain a mile or two MPG when I re-gear. I'll post again after the gears have been installed and I go through a couple tanks of gas.

Hope this helps!

Tony