Calling any 3.6L, 3.21gear, 315's users
 

So many of these threads out there, but none have directly answered my Q's. Recently got a 2013 JKU Sahara(3.6L, 3.21 gear) automatic which already had 285/65r18 Nitto Trail Grappler MTs and Rockstar II's, but since swapped out to 285/75r16 Duratrac and Procomp 7069's, and noticed a much smoother, peppier drive, and mpg+ 1-2 after procal flash.

I have Teraflex Leveling Kit installed, 2" in front, 1" in rear. My dilemma is I want to run 315/75r16's, but am hung up on a few issues. Fenders, performance, spacers, re-gear, additional components needed to preserve the jeeps longevity.

Few side comments first: It's my daily driver, and bring on the beach once every few weeks. I live in rather flat beach country in Long Island, don't see the highways too much. It will never see trails/rocks or tow, or be any where near full flex, which makes me think, based on what I have read, that I don't need to replace stock painted Sahara fenders or the stock tire carrier.

- Does the 3.21 3.6L have enough balls to push along 315/75r16's? I will be mostly flatland driving 25-50mph, but road tripping out west soon so highways are in my near future.
- Will I notice that much of a performance drop since I have never ridden stocks and have only known 33's? I've read that 2013+ 3.6L's have plenty of guts to push along 315's no problem and most hardly notice a difference city driving, as for 2012 and older 3.8L's is a different story. Acceleration off the line is of zero importance to me, and am not an aggressive or speedy driver in it. In my nova, that's a different story.
- Is highway driving 60-75mph rough?
- Are 35's going to cause any long term damage to any components or drive train? I don't plan on re-gearing, and have read I need to replace c gussets to give more strength to the axle. Is this necessary? As mentioned, I won't be on trails or crawling, and am concerned about the transmission long term as well. Any other essentials need to prevent premature wear?
- Current wheels are 4 backspacing, will I need 1.25"/1.5"/2" spacers? Currently no rub at full turns.
- How will stock Sahara fenders fare with the Teraflex leveling kit? Have read they clear fine, just need spacers with stock wheels or 4+ backspacing wheels, or else they rub at full turn. Really don't want to trim the painted fenders. As mentioned, I will never have these at anywhere near full flex, and my 285/75's clear by quite a bit right now.

Thanks for any and all insight!

I have the 6 speed 3.6l and run 315/70r17's with 3.21 gears and don't have a problem with it. No lift and flat fenders. When I need to pass someone on the highway or am traveling up a steep grade I have to downshift, but I expect to to that in any vehicle when going 75-80MPH. Would regearing help? Probably. Does a 3.6l with 3.21's have the "balls" to push 315's? Yes.

I am running Duratracs in 315/70/17 with 3.21 and an auto. I haven't noticed that much power loss and can drive highway at 80mph all day long. What I did hear was running 315's or 35's with 3.21 gears and an auto will put stress on your drive train, not sure how true this is. Also running larger tires will cause your ball joints and such to wear down a little faster.

I have a 2012 JKU with 35x12.5R17 BFG KO2s, with 3.21 gears and the 6 speed manual. I am definitely saving up for a regear.

-I think the 3.6l has enough power to move around 35s, but the 3.21 gearing limits it's ability to get that power to the tires. Around town and the flat interstate it's fine, but whenever I go up to the mountains of NC/TN I have to put the Jeep in 3rd gear just to keep going 45mph and I get passed by pretty much everything. I never use 6th gear anymore, and a strong gust is enough to take me from 75mph to 65mph if I'm not paying attention.

-I get ~14mpg city driving, ~17mpg mixed driving. It is my understanding that I'll only gain about 1mpg when going up to 4.56 gearing, but I'm not super concerned with MPGs.

-I am not a mechanic or engineer, so I can't say for sure that the larger tires and 3.21 gearing is good or bad for your drivetrain, but I can certainly feel mine straining more with the 35s, and if I can feel it straining I can't imagine it's good.

-As far as height goes, I think you're fine as long as you don't go to full flex.

If you haven't already committed to 16" wheels, I'd go 17. From all of my research, tires for 17" rims seemed cheaper (maybe not where you live?). Something to consider is braking. I've had a few close calls in my Jeep with 35s that I wouldn't have had with stock tires. As soon as funds allow (probably even before the regear) I plan on dropping some money into bigger rotors and better brake pads. I've seen lots of kits for 17"+ wheels, but I don't think you can get bigger rotors for 16" wheels.

I feel like very few people mention the necessity for a brake upgrade, so YMMV, but to me it seems like a necessity.

I've got a 2013 unlimited wrangler with 3.6L and auto tranny. 3.5" lift running 35"s. Had 3.21 gears for the first 18k miles since I bought new. Works fine on flat land, struggles on head winds and hills. Finally bit the bullet and re-geared it to 4.88 gears. Best thing ever, more response from throttle, more get up and go, brought back the power and torque. Gained 2-4 miles per gallon. Can run 75 or 80 mph on highway and never strain. Don't have to worry about gear jumping anymore. Best investment yet.

I have 37s and 3.21 gears with an auto. I wouldnt say it's untolerable but it's really flat where I live. It's not a rocket ship, but I dont try to drive like it is. It does down shift very often on the high way. I've off roaded a couple times including the Rubicon and it did just fine. Just took it easy on the throttle.

Just drove from detroit area to toronto and back and i kept it at 70 on the speedo. Only downshifts were on the inclines of overpasses and some of the hills in Canada. But the jeep felt gutless 50 mph and up. Usually stay under 55 anyway where i drive the jeep anyway.

3.21's with 35's? Bad idea. Yes - once you get going you can keep the speed but forget any decent acceleration or ability to climb up hill. Anyways with 35's you need at least 4.56's though 4.88's might be a better idea. With the lowest of the low stock gears you will end up putting too much strain on your drive train and your fuel economy will also suffer.

Your "road trips out West", especially once you get above 6,000 feet and encounter uphill grades will quickly sour you on the acceptability of 3.21s and 35"s. I'm running 33"S and 3.73s and am not happy in those conditions. Get up above 10,000 feet, forget it.