I've read on here people talking about not being able to use 6th gear on their 6spd JKs. Why is that? Is it because they are too low in the RPM range at 70-75? The 3.6 doesn't have enough uhmph to accelerate to or maintain highway speed in 6th gear? I'll be picking up my new-to-me '16 JK on Saturday and I'll have about a 600mi drive home on mostly interstates. I'm hoping I'll be able to maintain highway speed. Any info on what to expect would be appreciated.

***Update: Apparently you have to make at least 6 posts before you can upload a picture. Here's my JKU just waiting for me to pick her up!***

 

I’ve not read that but bet they are running significantly larger than stock tires, larger as in diameter which of course has the same effect of reducing your axle ratio as they travel significantly further per revolution.
Its my opinion that the average Jeep is running way oversized tires and lifts than they actually need, it’s done primarily for looks as the majority never see the severe use these mods are meant for, and these mods significantly reduce the road drivability and safety on the road.
A stock JK is more capable than my old CJ was and it was pretty capable. I think very few Jeeps that I see driving around actually need the tires they are running.

Then you have to define stock gear ratio as it varies by quite a bit, my 16 Rubi for example came with the optional 4.10’s which on stock sized tires in my opinion was a little over geared for just straight street driving, but with 33’s it seems again in my opinion to be pretty close to just right. But if it had 3.21’s it might be a little under geared. So if you have 3.21’s and run 35’s or larger, yeah I bet it won’t pull 6th gear.

Now mine is an auto, my preference was a manual, but I also only wanted a two door and didn’t want to buy new, so the number of available used good condition two doors was nearly zero, so I ended up with an Auto.

So in other words if it’s running close to stock sized tires you will be fine, Jeep didn’t build one that can’t maintain high gear not pulling heavy at highway speeds I’m sure, but if it’s got big ole tires then maybe you just need to run 5th, but consider that if the brakes are stock that you braking will be diminished too, unless of course the brakes were upgraded. To run big tires “properly” requires a whole bunch of money, it’s not just wheel spacers and a cheap lift.

 

I'm pretty sure mine came stock with 3.21 gears as it's the JKU Sport. I know it's sitting on 315/70R17 tires with aftermarket rims. It's also been lifted, but I can't tell from the pictures whether it's a 2.5, 3.5 or 4in lift. I also don't know if the PO went as far as regearing to accommodate the mods. I guess I'll find out on Saturday when I go pick it up.

I have been trying to upload a picture of it for reference, but the forum isn't letting me for some reason. I'll try to post it in another comment.

 

people want to slap larger tires on their JKs and that 3.21 gearing was just not deep enough to accommodate them. basically what you said here....at high speed the rpm range would be so low that when you put your foot down....there was nothing there. you'd basically wipe out the top gear in either an auto or manual. in the auto it was even worse cuz there'd be constant downshifting and gear searching. the slightest headwind would have you downshifting to 3rd just to maintain speed. low end is never the issue cuz people get used to just driving fat pigs around town.

 

Is there any way to identify whether the PO went to taller gears without having to take anything apart? Based on the research I've done so far, 4.10 or 4.56 seem to be the magic numbers with 35s. I have a whole slew of things I want to do to my Jeep, but first and foremost, I want to be able to drive it to work without having to constantly downshift to maintain speed.

 

315 x 70’s-17’s aren’t quite 35’s they are 34 3/8”. Not a bunch of difference but some. This site is excellent for scheming and comparing https://www.wheel-size.com/calc/?whe...scl=2in&sr=0in
Only way I know to tell is to jack the diff in the air and 3.21 will have the driveshaft turn 3.21 times for one exact rotation of the tire. Jack one of the front wheels off of the ground to count, both off due to the spider gears may muck up the rotation. Stock it’s on the data tag
Another way is to just drive it, if it drives fine who cares what the axle ratios are, lugging a motor isn’t particularly good for it, an advantage of an auto is unless you lock in a gear manually it won’t let you lug it, but excessive shifting will cause premature wear.
JK auto’s are I believe 5 speed so I assume you’re getting a manual? If so and it won’t pull 6th without lugging just run 5th. 3 speeds used to be enough, then four speeds came out and was the gold standard for ever, surely 5 gears is more than enough.
These things are what 285 HP or something? That’s a hot rod. My CJ7 was I think just under 100 HP and while it weighed at least a half ton lees it’s 4 sp was plenty.
In my opinion it takes a whole pot full of money to run 37’s or larger and do it safely and even then there is increased wear on some components.

TANSTASFL

 

it's pretty easy to notice just based on the RPMs when crusing at 70mph. google JK gear chart 3.6L and you'll see all the charts. measure your actual tire height and then the RPMs at that speed are going to pinpoint it for you without even having to spin the wheel and count driveshaft rotations. Odds are probably high that it has not be regarded. I've never experienced anything on 3.21 gears. from the start i was 35s and factory 3.73 and that was not agonizingly terrible, but i guess i would not want to experience 3.21a based on that. been on a whole slew of tire/gear combos since then.

 

My '77 CJ-7 was only a 3spd, and I4. It managed to run 65-70 no problem.

I don't plan on going to 37s or bigger. I'm happy with the way my JKU looks with the current set-up and I'm not going to be doing the type of off-roading that would require anything bigger. I would definitely like to look into regearing to 4.56 for the use of 6th gear as OD. My Son in law is already talking about an LS3 and TR-6060 swap in the future.....we'll see how that goes...

Thanks For the input. I definitely appreciate it.

 

What are your thoughts on 4.56 gears with 35s? Is that something you've run in the past?