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Old 10-29-2011, 09:26 PM   #1
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3.21 vs 3.73 MPG

Jeep #1
2012 2 door
Manual tranny
Soft top
225/75/16
3.21 gear ratio

Jeep #2
2012 2 door
Manual tranny
Soft top
225/75/16
3.73 gear ratio

Both driven the same, shifted the same, over the same terrain. Which is gonna win?

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Old 10-29-2011, 09:32 PM   #2
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3.21 no question. That's the whole reason they're there.

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Old 10-29-2011, 09:34 PM   #3
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Quote:
Originally Posted by daggo66 View Post
3.21 no question. That's the whole reason they're there.
Oh yeah?!?!?!?!?? we'll....ok
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Old 10-29-2011, 09:35 PM   #4
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But, how much ??
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Old 10-29-2011, 10:51 PM   #5
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I say the 3.21s win in the mpg department by some ridiculously stupid small amount. Half a mile more per gallon or less.

The irony is that I'm obviously willing to trade some mpgs for performance, as I've just stopped using 6th and always rev to at least 3k before shifting, often more. So I've wiped out whatever "advantage" in mpgs I get from numerically low gears by never shifting to them to capitalize in the opportunity. But hey, it looks good on paper and I have actually gotten 21 mpgs out of a tank in mixed driving with a 4 door. It just sucked and so I've never made the effort to drive that way again.
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Old 10-29-2011, 11:07 PM   #6
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Tom is right.

The 3.21s will give about ten to fifteen percent better than the 3.73s given the parameters you cited above but naturally also assuming the same RPM shift points and acceleration / deceleration and cruise speed.

If it were an automatic trans, then a whole different set of assumptions weirds up the thinking.
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Old 10-29-2011, 11:46 PM   #7
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I have personal experience with vehicles in the past, not the JK, where I got better mileage with 355's over 308's that came in a vehicle. The 308's had the engine just bogging. The lower gears allowed it to operate in the best place of its powerband and thus best mileage.
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Old 10-30-2011, 12:05 AM   #8
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3.73s should come standard for any 6 cylinder jeep.
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Old 10-30-2011, 01:03 AM   #9
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I think you would get better MPG's with the 3.73's. IMO these 3.8's need to rev a bit more than the 21's allow. But I hated my 21's FWIW. Oh ya, they were great when I was going downhill with a horrific tailwind.
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Old 10-30-2011, 02:26 AM   #10
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It would depend on the terrain....
But since both have the 3.6's (2012's)....and both are manuals...I think the difference would be pretty small below 50mph. Since you could control your shift points...there isn't really an advantage in having 3.21's.

However...at 70mph in 6th gear, there difference in RPM's turning is pretty big.
3.21's = 2055 vs 3.73's = 2387
If you spend a lot of time on fairly level highway, and you don't plan on heavy towing or modding...the 3.21's would be the winner.
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Old 10-30-2011, 08:43 AM   #11
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Quote:
Originally Posted by panthermark View Post
It would depend on the terrain....
But since both have the 3.6's (2012's)....and both are manuals...I think the difference would be pretty small below 50mph. Since you could control your shift points...there isn't really an advantage in having 3.21's.

However...at 70mph in 6th gear, there difference in RPM's turning is pretty big.
3.21's = 2055 vs 3.73's = 2387
If you spend a lot of time on fairly level highway, and you don't plan on heavy towing or modding...the 3.21's would be the winner.
YUP!!! Rev control, whether you need less rpms (more MPGs) or more rpms (power now)
*Now if they had a U.S. CRD engine option, you would want the 3.21 gears for perfect balance.*
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Old 10-30-2011, 09:29 AM   #12
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From reading this and other posts, it appears that:
- the 3.73 has more torque 0-10 mph
- the 3.21 gets better gas mileage at cruise in top gear
and in between it depends upon your shift points. Now quantifying the benefits of each seems tricky. I am surprised that Chrysler did not have to rate the mpg of the 3.73 and 4.10 separately from the 3.21. Seems like the mpg rating could be a little bait and switch.
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Old 10-30-2011, 09:37 AM   #13
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I'm getting pretty darned god mileage from mine- well, at least better than I feared. Steady 20-21mpg in mixed driving with the 3.73.

It seems like it doesn't have to hunt as much, and more gets done at lower revs.

However, let me get back to you all once I'm done with my paper mache nosecone.
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Old 10-30-2011, 09:42 AM   #14
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Same numbers for me XPA in DC area traffic. I am pleasantly surprised by my fuel mileage as it isn't hardly any lower than my mustang which I traded for it. I was expecting 18 or 19mpg with the 3.73's but am pulling about 20-21mpg so far and am glad I went with the 3.73's instead.
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Old 01-14-2012, 12:17 AM   #15
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I'm in the DC area and do the Rt 66 traffic twice a day. Just got my Sahara Unlimited 4 weeks ago and have the 3.21's and have been thinking about a gear change already. I've noticed she downshifts far more frequently that I'm used to, but then again my other car is an 05 300C 5.7. My bigger question would be 3.73 or 4.1? Eventually would like to go with 35's anyway so I think it would be a wash.
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Old 05-19-2012, 01:52 PM   #16
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What about downsizing tires?

Quote:
Originally Posted by Celtic View Post
Tom is right.

The 3.21s will give about ten to fifteen percent better than the 3.73s given the parameters you cited above but naturally also assuming the same RPM shift points and acceleration / deceleration and cruise speed.

If it were an automatic trans, then a whole different set of assumptions weirds up the thinking.
Have 1,300 miles on a new 2012 Sport S with 3.73 and 6-speed. Mostly around town/highway in Colorado/Utah, but some weekend warrior stuff to get farther up some old mining road to a trailhead (too much for husband's Patriot 5-speed). I like it (Dozer is sharp) but wish it road smoother drove easier around town (most of its use). Also wish it were a bit lower for easier ingress/egress (I'm 5'3"). Maybe I should have bought a Sport with 3.21 and smaller tires, but the dealer didn't have one -- and I'm nowhere near a big enough town to do comparison driving (I know, I should have driven over to Denver). What happens if I downsize to the size tires on the stock Sport? Higher RMP? Worse gas mileage?
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Old 05-19-2012, 02:08 PM   #17
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Freeway or highway driving the 3.21's will never hold 6th at 60 or under unless it is pure flat.... My 6spd with 3.73 will hold 6th gear from 55 on up, over hill and dale. So, while I am cruizing along in 6th the 3.21's will be downshifting, upshifting, or just leaving it in 5th. My milage will be better and easier to drive. Around town, my 3.73 will stay in a higher gear then the 3.21 and I will get better gas milage, again. Plus, it is a lot more fun to drive a 6spd with the 3.73 then the other one. Now, off road, the 3.21 might as well stay home, especially with any bigger tire size. From my viewpoint, the only reason to have the 3.21 is if you drive at 70 or above and who cares about gas milage at that speed.
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Old 05-19-2012, 02:22 PM   #18
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When I was in high school, many moons ago, camaros and mustangs ran 3:21 gears. To imagine that in a Jeep is really depressing.
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Old 05-19-2012, 02:30 PM   #19
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I will give the advantage to the 3.73's. This is only if you don't have to upgrade the carrier when switch them out for 4.88's, thus saving you 100 bucks in new carrier costs at some future point vs getting new carriers with the 3.21's.

that and it will give you a lower crawl ratio off road...
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Old 05-19-2012, 03:04 PM   #20
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3.21 or 3.73 ?? good read

hi, well here goes, i had a jeep 07jk 2door 3.73 got 13 mpg's
now have 2012jk auto lsd 3.21 and get 20 to 30 mpg's i do drive
95% on road, when i ordered my jeep the 3.73 was cheap $50. but
i got my fill of 13mpgs for 5 years, but thats what's good about
jeeps, u order them the way u want, i also got the power package
and some jeep owners want crank windows, so jeeps, get it the way
that fits u best....well have fun every day.. when gas goes down
to $2.00 i will get a 4.11 rear end. you think we will see $2.00??
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Old 05-19-2012, 03:26 PM   #21
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I have an 07 2door with 4.10 rolling 35s, I avg. 18 mpg. If I avoid doing 70 and stay at 55 or 60mph then I get 19.5 to 21. Forgot to say it is a manual
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Old 05-19-2012, 03:52 PM   #22
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I have a 12 with a manual and 3.21s and I get over 21 quite often. If I were to do it again I would probably get 3.73s
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Old 05-19-2012, 06:00 PM   #23
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I wouldn't even consider getting the 3.21 on a jeep. I've had weak gears on jeeps before and if you want to get bigger tires down the road 3.21 gears will rear their ugly head even more. 3.73 gets would be mandatory on any jeep of mine and 4.10 would be even better.
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Old 05-19-2012, 06:17 PM   #24
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It depends on how you drive. 3.73 gears will give you better city mileage. The engine doesn't have to work as hard in stop and go traffic while the 3.21 will do better on FLAT hwy were you don't have to downshift but if the hwy is hilly then the 3.73 will do better again because it doesn't have to downshift as much.

Really 3.21 is worthless gears. Sure you can get by with them but the 3.73 is so much better overall.

I say this because I regeared and by going up in gears I got about .5 mpg better overall gas mileage. Being that this was the same vehicle it showed how mpg's effected gas mile. Of course this was 3.73 going up to 4.88 but it is the same principle. The 4.88 just made the engine work less at speeds under 50 mph were most people drive the majority of the time.

Now over 60 mph, I lost about .6 mpg but the gains at the lower speeds more then compensated.

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