gjoyce3

It is decision time on which gears for my daily driver 2012JKU Rubicon with the Automatic transmission. I put a 3" Teraflex lift on to fit 35" Toyo M/T's. That coupled with the front and rear bumpers, winch, and skid plates seem to have made for a very heavy JKU. Around town (San Diego) it drives just fine and gets up to speed on the highway just fine - not a peppy as it used to be, but acceptable. However, when I go up a hill it has to dig deep into the gears and has difficulty maintaining speed. At 70mph, the tach reads around 21K. It will get up to 26k if the road goes up a grade. MPG also sucks ~13mpg.

The charts would indicate that a 4.56 gear swap is in order. When I scheduled the work, the shop strongly encouraged me to use 4.88s. This has me dithering. One reason put forth is that the evolution path of jeep owners inevitably leads to 37" tire. Frankly, I do not see this happening. My primary goal is to get it back to speed on the hills and I am hoping that some of the poor gas mileage is due to lugging. I plan on several off-road trips per year and am currently getting ready for the EJS. Looking for advise.

Thanks!

gjoyce3

jaybrown

Auto get 5.13 s. 4.88 for the 6 spd manual

jaybrown

4.56 you can do because of the better engine

dnorrell

I personally feel you would do just fine with the 4.56 with the changes they have made to the '12 powertrain. I have driven a friends auto with stock 4.10 and 35s and I wasn't really wanting for power, and he was still seeing decent MPGs around town.

wheelman2925

The overdrive(.83) for the new 5 speed gear box is shorter than the older 4 speed(.69). The gearing scenarios for the old powertrain simply do not apply. 4.56 gears with 35' tires at 65 mph will yield 2362 rpm whereas 4.88 gears will turn 2527. These numbers are based at only 65 mph. If you do freeway travel where speeds are routinely above 70 the rpm's get pretty high and mpg will suffer accordingly.

FWIW 4.10's and the stock tires works out about the same as 35's w/ 4.56. First gear in the wa580 is lower as well (3.59) versus the 42rle(2.84). So even with numerically lower 4.56 the '12 jk will pull more rpm at crawl esp w/ the rubi's 4:1 low.

Good Luck

JPop

I concur with Wheelman, 4.56s are the way to go on a 2012. You could probably make a good argument for 4.30s if they were available. There also isn't going to be the same efficiency robbing downshifts out of overdrive of the old transmission with increases in rpm of over 40% and the new transmission with just 20%.

gjoyce3

Thanks a lot for your feedback. I am feeling a little more secure in sticking with the 4.56's.

gjoyce3

gjoyce3

Well apparently, the demand for 4.56 gears has taken the suppliers and shops by surprise. Two different installers were unable to locate a set of gears and at best secured promises for a month out. One of the on-line vendors that I contacted confirmed the situation. Unless someone knows of a steady source of quality 4.56 gears for front and rear, I will have to cool my jets

gjoyce3

Nwapache

Have you tried David at Northridge ? I bet you a beer he's got what you need

jeepgrnjk

Just replaced my gears....hard time getting 4.56 for front back ordered till mid April. Had to go with 4.88 due to issues with getting wrong gears for front then found out they were back ordered. Wait and go with 4.56, 4.88 are great but at 65 I'm running at about 2750 rpm in 6th gear with 35" tires.