Captain Wild Thing

I have had 321 when jk was stock with 32 and managed 24mpg then I stuck 35 on and only got 15mpg just stuck 373 gears in haven't been able to get an mpg yet but can tell it has better pickup and go than 321 gears. I have done the hardest trails in big bear ca with 321 gears no lockers just fine. But everyone calls me crazy.

KTuckyJK

I have 3:21's with 3.5" lift and 35" bfg km2's. It is not a race car but is totally drivable. I also have a superchips programmer so that helps a lot. I get about 19 mpg's on my daily 100 mile round trip to work which is almost all four lane highway running 65 mph. I hit the trails about twice a month and have no problems getting around.

bkjeep

Oh you have a 2 dr! Your commute must be dead flat. I'm getting about 14. Do you think the super chip helped much?

dan3jeeps

If I am figuring the final 4 low drive ratio correctly, all else being equal, the 3.21 axle gearing gets 38.9 : 1 while the 3:73 axle nets a 45.1 : 1 for only a 6.2 revolution difference, or a 17.54 %. Does not seem like a large enough difference numerically to make a huge difference in terms of applying power/torque to the ground. But maybe that is all that is needed to move the engine's powerband up just enough into a more usable range?

By putting somewhat taller tires on a JK with the 3.73 axle gear pretty much takes us to the equivelant of a stock JK with the 3.21 gear and stock size tires (all else being the same). Right? More or less?

JKlad

Copy & Paste this link into your browser window and have a look. You type in the stock tyre size & then the different (bigger or smaller) size in either imperial or metric & it gives you both the change in the spedo accuracy & also the different revolutions per mile over stock & also a graphic comparing the two. Will answer your question:

http://www.miata.net/garage/tirecalc.html

mpco239

I have right at a thousand miles on my JK Unlimited auto, 3.73 (3.21 six speed equivalent?). My impressions are as follows: Interstate- Jeep does pretty good. Downshifts on some big or long hills, but over all not bad. Around town- Jeep does pretty good. Nice low end with the 3.73s and factory 17s, (31"?). State/local higways with a 45 mph limit- Jeep has its biggest problems with these. In OD, its barely over 1000 RPM. As long as its flat, it does alright, not the best throttle response. Even slight hills force you to make it downshift because of how much you have to open the throttle. I think the 3.8 and the 3.73s are a good combo with the auto. I cant complain a whole lot. My main concern is the trans being in a bind in high gear creating some heat. I put in a banks CAI to give it a little help, especially since I plan on doing some moderate towing. Only had it on about 20 miles, but throttle response seems to be better. We shall see how it works out overall. The 3.8 seems to be a pretty good engine. The Jeep Unlimited is pretty heavy, and the pentastar should be a great improvement. My wife has it in a 2011 Grand Caravan R/T and it pushes that 4500lb beast around very well. If the 3.73s with an auto are equivalent to 3.21s with a six speed, then I would say 3.21s are probably absolutely adequate, but by no means ideal. I suspect the 3.21s will be much better with the 3.6. All that being said, I am by no means a hard core jeeper, and I am very happy with the stock 31s (although not the actual tire....can a brother get a real all terrain on a trail rated vehicle?) as well as the clearance. I can see a re gear being almost mandatory if you get much taller with the tires.

Poser

Sucks! JKU is too heavy for 6speed 3.21 w/32" tires. Can't even go up small hills in 5th.
Wheeling in 4lo, not bad for mild trails, I've only done mild and it did kinda ok. If you get a really good deal on a used unit, get it geared! I'm getting mine done very soon and I can't wait. JKU w/2 adults + 3 kids & 32" tires is dumb. But that's my opinion.

Go for a test drive...

jhol111964

My 6 speed 3.21 with the stock 29"s was fine, and was getting around 19-20 mpg. I put stock Rubicon BFG MT 32"s on recently. Much better balanced look, but my mileage dropped to 16 mpg. I know the rolling resistance of the MT's is greater, plus it sits a bit higher now increasing drag under the chassis, and, I've taken the hardtop off for the summer, and I think that increases drag too. And, 6th gear is almost useless now, slowing down and not pulling hills without downshifting. It cruises at 75 mph much better in 5th with the top off so 5th is my highway gear. With the top on, 6th gear is more useful, so the top significantly affects drag at highway speeds.

More importantly, It recently, like in the last 2-3 weeks, has significantly increased in power and acceleration. I heard the computer regulates power output during the first few thousand miles as sort of a breakin period, and eventually tapers off the power reduction. So with the 3.21, 32" tires, its pretty darn quick around town if you use a lot of gas pedal.

If you have a choice, I'd go for the 3.73s. But if there's a good deal on a 6-speed 3.21, and you're not trying to drag race, you'll be happy with it.

barnettd

I put 32s on mine and computer mileage dropped but the increased tore diameter is the biggest culprit to "lower" mpg. Check your odometer against a. Gps while traveling a known route and see the difference

jhol111964

Right. The diameter difference is 10%, but my mileage has dropped by 20%, so I'm still loosing 10%. That gets me back to 17-18 mpg. That's fine though, I didn't by it for a gas mizer.