So I just got my Teraflex 2.5 coil lift and 35" Duratracs put on my new 2011 JK unlimited and love it!

However I can definitely tell I will need to regear in the future. Driving around the city seems fine but the noticeable change comes when driving on the highway. The lack of power (not that there was that much before) makes it very hard to maintain highway speeds without constant gear switching while OD is on.

I have noticed in a lot of the gearing threads people recommend to turn OD off with 35" tires if you haven't regeared yet. What does that accomplish and would that help with highway driving? Would turning OD off help with highway fuel mileage by maintaining a consistent RPM without the constant gear switching?

And just FYI my current gears are the 3.73 with LSD and it's an auto if that makes any difference.

Thanks for helping out the newbie!

 

every jeep and situation is different. you being in texas and depending on your road habits will be the only real truth.

it will help with over drive off because it isnt constantly hunting for gears.

you will also notice that your speedometer will say you are going 70 on the freeway and you're really going about 80 and your rpms with the over drive off will be near 3k.

 

I'm a newbie as well (1st post!) and still researching my lift/tire combo.

I beleive that by turning off OD, it just eliminates the OD gear which is made for low RPM highway cruising and good gas milage. Upon installing your bigger tires, the computer and gearing make your Jeep think it's going slower than it really is.

By elimnating the OD gear, your run higher RPMs which will give you a touch of better performance until you re-gear.

**Newbie alert**Please correct any and all mis-information!

 

Turning O/D off basically keeps you in 3rd gear as the top gear instead of switching back and forth all the way to 4th. This'll make you run higher rpms than with O/D on, BUT, it won't have to downshift every time you hit a little incline and this saves some on the transmission life. I don't foresee any gains in fuel economy by running constant rpms, because compared to 4th gear, you'll be running higher rpms, so what's gained on one end is lost on the other.

You hadn't mentioned if you use a programmer such as Hypertech or Superchips to recalibrate your speedometer and adjust your shift points for the automatic? If you haven't done that yet, it will definitely help and you'll notice that it doesn't feel as sluggish.

 

Thanks for the help, that definitely makes sense. I wasn't worried too much about fuel milage really, just speculating.

I don't have the superchips programmer but have heard that it helps so it's definitely on the "list". If the funds for a regear don't come quickly then I might fork over the cash for the programmer first, since it's cheaper, then save for the gears.

 

Even if you re-gear, you'll still need the programmer to program the computer for the gear changes, etc. So, definitely something you'd want in the near future. For the moment, if you wanted an inexpensive alternative, get the Pro Comp Accu Pro or Hypertech Max Energy to just recalibrate your speedometer for the tire size change and eventually when you're ready to regear or go for a Superchips, sell the one you have and upgrade to the Superchips. After going to 35s, for the one day I had to wait before my Hypertech came in, my Jeep felt like a pig... super sluggish, and after re-calibrating my speedometer (no power programming for premium or anything else), it felt rejuvenated again.

 

Overdrive knocks out the last gear and raises the rpm shifting. Helps keep a higher rpm.

 

i had the exact setup for about 6 weeks before going to 5.13's. 35" duratracs, auto and 3.73's. i would put on a tuner and get your speedometer and tach correct first. i was coming off 33's with no tuner. my mileage on the highway actually went up after the tuner and 35's were installed. i used to get in the jeep, turn off the OD and drive really slowly wth the 35's while i waited to get the gears done. the performance wasn;t that bad (not good either though). the tranny however would run around 185 and up all of the time. i think a cooler would be the next thing to install after the tuner (and then the gears).

 

Have you found the FAQ's yet? (stickied to the top of this modified area)

Open the drivetrain section and look at the RPM chart.

Here is the OD info:
Some assumptions:
-The stock 32" tires actually measure 31"
-Your 35's actually measure 34"
-From many, many other threads, we know that this engine likes to run in the 2400-2700ish rpm range. (best power/efficiency/whatever the correct word is)


auto, 3.73, 31" tires
1953rpm @ 70mph (OD On)
2830 rpm @ 70mph (OD Off)

auto, 3.73, 34" tires
1781 rpm @ 70mph (OD On)
2581 rpm @ 70 mph (OD Off)

auto, 5.13, 34" tires
2450 rpm @ 70mph (OD On)

auto, 5.38, 34" tires
2569 rpm @ 70mph (OD On)

For comparison:
manual, 3.73, 29" tire
2542 rpm @ 70mph

manual, 5.13, 40" tire
2535 rpm @ 70mph


So, just from the numbers:
-Auto's are under-geared straight from the factory.
-With the same gearing, Manuals need to run a tire that is 5" taller to turn the same rpm as an auto.


*My opinions and lousy math skills. Feel free to play around with the numbers and see what you come up with...

 

I get an average of 19.5 mpg with the stock 32" tires at 75mph and 2000rpm on the highway (OD on) what can I expect with 35" tires. I have a 2011 with 3.73 gearing and I live in Florida where it's flat for the most part with the occasional rolling hill, but rarely does the transmission downshift from 4th. Every vehicle I've ever owned including my last pickup truck has cruised at 75mph at around 2000 rpm. Call me crazy but seems like the lower the rpm the better as long as you live in a flat area like I do. If I go 35s and my RPM drops to around 1700RPM at 70MPH is that such a bad thing as long as I am in flat Florida?
I hear guys talking about their mpg going down to 12mpg on the highway and seems to me it's because they are running at 2500+ RPM or more. I guess I just don't have a lead foot and notice it like a lot of people.

I will eventually re-gear when I get the cash to do so, but tires have to come first and my hope is with a programmer I'll be able to get by as others have already suggested. I am also guessing that smaller and lighter 35 tires probably make a difference as well and could help.