nrhoney

So I'm a little worried... I just got a 2013 Rubicon (2dr) automatic (i'm pretty sure it's geared at 3.73) with 35" (Goodyear Wrangler w/ Kevlar). I've got a heavy front & rear bumper (total maybe 200 lbs, Rugged Ridge XHD but no winch) and a 4" lift. My Jeep looks perfect, it's exactly what I wanted but my mpgs SUCK, 11-13. It runs fine around town (it could have more pick-up) but on the highway RPMs jump all over the place, mainly with the cruise control (bounces between 3-4k RPMs north of 70 mph). Do I need to gear up? I'm really new to all this. My techs at the dealership are great but I'm getting mixed messages. What am I doing wrong and/or what should I change?

rays0852

Many people find 3.73s and the new auto to be OK with 35s. However if you are having issues holding gear at hwy speeds and up inclines, you might want to consider re gearing.

I would start taking measurements of your RPMs at various speeds. Keep in mind....75plus mph in a jeep with a 4 in lift is asking a lot. The new engine and trams are good, but pushing that much air resistance will make it hard to hold the gear with what you have. However if you go 4.56 I think you might be a little over geared and be right back at 3k at 75mph. It's a hard sweet spot to find. Hence the fact that some guys are opting for V 8 conversions. The higher torque motors seems to hold a gear at a lower RPM than our Pentastar.

jkbarnett

Have a question regarding measuring tire size.

If you lower the tire pressure, the over all diameter will decrease but the tire will still travel the same distance... right

Doesn't one need to know how far the tire will roll per revolution ?? and use this to determine tire size ??

grady

Something doesn't sound right. I have 13 Rubi auto still with stock tire. At 70 to 75 mph I am only turning about 2300 to 2500 max. With oversize tires you should be turning less rpm than me since you get more rollout and speed than I do. You should have less bottom end this the point of rehearing to get your Roma back UP into the power curve for better performance and mpg. With lower gears your Roma go up and you said you are already near 4000.

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nrhoney

I agree something doesn't seem right which is what prompted my post. I know I'm pushing a ton of head-wind at 75+, which is most of the problem. I need to test it more, I've only had the truck 2 weeks and only gone high speeds on the interstate 3-4 times. I think it's trying to run in lower RPMs but can't maintain the speed so it's dropping gears and revin' RPMs to pull the speed back up. At or above 70 mph the cruise control just can't keep up. The speed ranges +/-5 mph from where I set it. I would regear if it helped but seems kind of pointless to spend the money to go from 3.73 to 4.10 but 4.56 might be too much. How much should something like this cost anyway? $200-300 parts? + $200-400 labor? The little off-roading I've done so far, Blubic handled flawlessly. I would be regearing only to help city acceleration and highway power/control. Thoughts?

kh202

Did you recalibrate with a Procal after you got your bigger tires. That's important to do with an auto. It resets the shift points. Your computer is probably confused right now

nrhoney

The dealer said they did, I know they tried. But I'm not sure it actually took. Is that something I can do myself or is it more likely I'll screw something up? The Recon manager, who I've become friends with, has basically the same set-up (lift, 35"s, etc) as mine but on an Unlimited so I'm sure they have a clue what their doing. He let me drive his and his was a little smoother but he attibuted the difference to the longer length of the Unlimited.

kh202

I thought the dealer can only recalibrate up to 32's, but i could be wrong. Do you have a smartphone? You can just pull up your gps and compare speed on there vs what your speedo says. For me, my speedo says 60 and actual is 64-65, but I have a manual, so it doesn't affect driving at all. And it's pretty easy to do yourself, but you need to buy a procal or similar unit. Not something you can just adjust.

rays0852

Yes you are right. That would be the most accurate method.