So I finally found a great deal on (6) like new duratrac 315/70/17 tires for my 13 JKU automatic with 3.21 gears. Ive almost chickened out a couple times due to all negative I have read about the power loss and such. I got them mounted yesterday and went on a 200 mile trip today. It was all interstate here in central texas. I set my cruise on 68 and was turning right at 1800 rpm's. I must say, I was shocked. On flat terrain, my ECO would stay on and speed maintained. On slight inclines, the ECO would go off, but speed was maintained and transmission did not shift. On a few of larger inclines, it grabbed another gear. I would say over the 200 mile trip, it downshifted to about 3000 rpm's 3 or 4 times. I was expecting it to be downshifting on the least little incline. Now, im confused whether I should regear? 99% of my driving is pavement. I guess the biggest concern is, will staying 3.21 gears and downshifting occasionally have negative impact on transmission and such in long run..? I know it could have better power, but its tolerabe for my driving style. I just don't want to damage anything by not regrearing. I also had a full tank of gas and (5) 285 terra grapps in jeep with me. So, there was a little extra weight. I had planned on regearing to 4.56, but now im not 100% sure. Any thoughts or related experiences?

 

First, Duratracs are some of the lightest tires out there so the weight increase is not really an issue. Second, regearing miss really about your personal experience and what performance you want out of your Jeep with your driving style. If you're happy, don't spend the money. On relatively lat terrain on pavement you won't have big issues.

Now, I have a 2013 JKU with heavier mud tires in 35" and I went from 3.21 to 4.56. But I was looking for serious performance off road as well as good drive ability on road. The 3.21s didn't cut it for me.

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i've got a 2013 jku auto with the same size tires and 4.56 and am very happy with it, but i will say that it feels very comfortable on the freeway at about 60mph and at 65mph it is spinning right around 2,500rpm... i pretty much get passed by everyone on the freeway, but that's ok as i've got the top down and am enjoying the sunshine. and as an added bonus i don't have to worry about getting a speeding ticket.

i don't have any regrets with mine. but if you don't have any issues with your current setup, i'd probably stick with it as regearing isn''t exactly a cheap upgrade.

 

Will there be any potential negative long term effects on transmission or other components if I dont regear? Im not a speed demon and pretty much try and keep my rpms under 2000 from takeoff to cruising speed.

 

Did you recalibrate your speedo? I got the same size tires with 3.73 w/auto I don't think the performance is bad but I live in mountains and I think 4.10s would be ideal with that size tire

 

I don't think so. I have 33"s (toyo open country at2's) on my 2013 jku with auto and 3.21 and live in the mountains and I can chirp the tires taking off from a dead start. I have read so many threads about how bad the 3.21's are but I don't find that to be the case. I have test drove same vehicle with 3.73's and could precieve no noticable seat of the pants difference and I have the speedo calibrated with the pro cal. I do wish I had a better crawl ratio in 4lo. I think I am going to forgo a gear change and do the transfer case swap.

 

I've got 3.21's on a 2012 Sahara unlimited auto running toyo open country mts 35 x 12.50 x 18 and love it! Averaging 17 mpg calibrated my speedo with a flashcal! I felt the same way and wanted to regear until I got my flashcal.

 

If you wheel it in 4hi with 35s and 3.21 gears you can pretty easily overheat an auto tranny. I did it with 3.73 gears (13' JKUR) because I just wasn't paying attention. Should have been in 4lo anyway. Just be aware that heat can be an issue if you have to slip the torque converter, ie rev the engine in gear without moving or just moving very slowly. When I over heated my tranny, I had to let it cool briefly before the jeep would move. Not great for the tranny.

 

In my experience, 4:10's are perfect all around for 35 and up. Lower the gear (bigger number) if you want more low end performance.
3:73 with 35's never hurt anything for me either but the change to 4:10 made a significant power increase. With a higher gear(lower number) you should get worse mileage around town but higher speeds per RPM on the highway.
And like mentioned, if you feel the need for more power or auto slipping during your limited off road use.....just switch to 4-Low and call it a day.

Currently I run 3:21 with 33's. No issues what so ever. Plenty of tire scratching when I want it.

 

I believe some 3.6L just run better then others and if you get a good running one 3.21's are fine. It may be ecm related. I don't know what else would explain why some with 3.21 gearing are peppy off the line and others are not. I know I fully expected mine to be doggy when I went to 33's from the stock 29's and was very surprised that it still felt great. I live in an area where you either are going up or going down so not a flat lander.