Jk overdrive questions
 

I am looking to take my jk on about a 1500 mile round trip soon and had some questions about overdrive. I am lifted 4.5 inches with one tons and 5.38 gears with 40x15.50x20 Toyo m/t tires. Going 65-70 in o/d I’m around 2100 rpm but will not maintain speed unless on completely level ground. When o/d is off I’m turning about 2800-3000 rpm going 65-70. I know my mileage will be terrible but will running with o/d off for that long hurt anything? I will run with o/d on when the roads will allow it but will running it with o/d off for that many miles hurt my tranny?

You should be fine. I have 3.73's and 35's and I run with overdrive off all the time. According to the gearing charts this puts me in the same RPM range as if I had 5.13's gears, which is what I need with my 3.8 v6.

Given the tire size, I think I'd be more worried about the transmission when driving OD/On. How often does it typically drop in and out of OD?.

With 37's and 5.38's, I have gone 10+ hours towing a boat or storage trailer from Austin to Denver, most of it OD/off. Other than poor mpg's, I don't think these 3.8's mind a little higher rpms.

If on fairly level roads it's doesn't drop out of O/D very often. It only drops out of O/D when I get to small hills and inclines. But I am actually pretty surprised at how well that small 3.8 does pulling those tires that weigh 109 pounds each. I have considered a sprintex supercharger as well but not sure how the engine will hold up to it.

If you have the 3.8L, then I would consider adding a trans cooler for added piece of mind if you haven't done so already. That is one downside of the 3.8L. In some not so wise decision making at FCA, they decided the trans cooler was not needed as a standard part on the 2007+ Wrangler although previous gen Wranglers had it. Many have seen the "hot oil" dash light come on when traveling on roads with hills and when climbing hills off road. FCA added the trans cooler back in as a standard part on the 2012+ Wranglers along with the new engine and trans. It is also believed that some of the fires on the JK on the 2007-2011 JK's without the trans cooler have been due to overheating transmission fluid. FCA obviously will not admit this is the case until someone is able to provide indisputable evidence, but in knowing you'll be traveling at higher than usual RPMs and turning OD off, and likely climbing some on road hills, it would seem like a really good idea to have a trans cooler installed. FCA issued a recall awhile back, where instead of fixing the hot transmission fluid problem, their fix was to turn the dash light on sooner. The Mopar trans cooler is one of the better quality ones, and most likely should have been there from the factory, and ironically looks like a factory install.

I'll be getting one and installing one shortly. I'm currently running 3.73 gears with my 3.8L auto and 35s. 30 minutes into an off road sand dune trip climbing some hills and my "Hot Oil" dash light was already coming on. The Hot Oil light was confusing, because it's actually a warning light for Hot transmission fluid.

Do a Google search on "JK Wrangler Hot Oil Warning" and you will find many discussions on this forum as well as others.

Some reference discussions:
https://www.jk-forum.com/forums/modi...arning-334584/
https://www.jk-forum.com/forums/stoc...cooler-302724/
https://www.jk-forum.com/forums/modi...ed-fix-234847/
https://www.jk-forum.com/forums/stoc...il-fix-162023/

The 2007 through 2011 automatic JKs do have a transmission cooler. It's too small, and being at the bottom of the A/C condenser, it doesn't get much of the air that passes through the grill.

When you install your aux trans cooler, it's the OEM cooler lines you will be tapping into.

I do plan on adding an auxiliary trans cooler soon. I have been looking into the b&m coolers. Any input on those coolers?

B&M coolers are popular. There are a bunch of reviews write-ups on the site, and some videos on youtube, easy enough to find them if you need a little install guidance.

This one is in the Write-ups area (there is an index stuck toward the top of that section.) It's the cooler I've had installed since 2008.
https://www.jk-forum.com/forums/jk-w...-cooler-37839/