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Discussion Starter #1
This is not a scientific experiment, but.......

I just ran the same 10 miles of inter-city interstate at a cruise-contolled 65 mph. Ten miles in one direction, turned around and ten miles back. I re-set the "Avg Mpg" on the dash as I started both runs. The first run was with the auto trans in the "D" position, so it was in 5th gear. The return run I locked out 5th and ran it in 4th gear.

First run 10 miles @ 65mph (2000rpm in 5th) = 23.7MPG
Rtn run 10 miles @ 65mph (2300rpm in 4th) = 22.6MPG

So, in simple non-scientific terms, on this test at the same road speed, but differing engine speed (rpm) the lower rpm generated a fuel economy improvement of 9.5% (1.1mpg). Might not sound like much, but if you run a tank a week like I do, that equates to an additional 20.46 miles per tank @ $3.30 gal is close to $325/year in savings.

Granted, we don't buy Wranglers for good fuel economy, but I bring this up to simply show that even slight variables in engine speed can make a difference in fuel consumption; something else to consider when we start talking gear ratios and tire sizes. The 3.21 works great for me, and considering that I won't upgrade to anything bigger than a 33" (not much bigger than the stock 255s) I think I made the right choice FOR ME.
 

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Nothing wrong with 3.21s on the new auto. Especially if you plan on staying stock and you don't live near a lot of hills or do a lot of towing. Even more so for a 2 door.

I would still prefer 3.55's for my JKU (which will stay mostly stock)....but they are not offered, and I wanted the 3500 pound tow rating....so 3.73's it is.
 

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Cool.

That is about what I would've guessed--that 3.21s net you about 1 mpg better than 3.73s.

Of course, on a $25k to $35k vehicle, I just can't get behind compromising performance for a measly $375 savings in gas per year. But that's just me.
 

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highway only mileage is not realistic, though. you need to factor in stop and go, city, and hills. 3.21s will be worse off below highway speeds and through hills.
 

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too many varibles to come to a conclusion based on 10 miles one way. wind, elevation, temp are just a few of the things that could be different. I however have the same set up and think it works great and most definetly keep it in D when on the highway for most efficient running.
 

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A 5,000lb WK2 Grand with a whopping :eek: 3.06:1 axle ratio and the same W5A580 (NAG1) auto trans can tow up to 5000 lbs, so the 3.21 on a JK is not going to be a limiting factor in any situation other than for someone running 35" tires on very hard trails, IMHO. The 3.21s feel very good in the auto equipped Pentastar JKs and push a JK well enough off road on 33" tires that I don't understand other threads worrying about it. The 3.73s add just a bit more shove on launch, minor help to the auto, but better for manual trans for easier take offs (regardless of manual's better 1st gear).
 

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A 5,000lb WK2 Grand with a whopping :eek: 3.06:1 axle ratio and the same W5A580 (NAG1) auto trans can tow up to 5000 lbs . . . .
Too be clear, the new grand cherokee comes with 30" tires. :whistling:

That said, I've got 3.21s in a 2010 with the 3.8 and a manual transmission with 33x12.5R15s and have no problems. And I commute 70 miles a day.

People who freak out about 3.21s being "okay" or "not working" are alarmist and foolish. 3.21s will work fine. Even for 33s and even pre-2012.

The question isn't really whether it'll work, the question is if it's the BEST CHOICE.

For 32" tires and up in any JK, I don't think 3.21s are ever the best choice, really. But they'll work just fine.
 

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I live at 6600 feet in the Rocky Mountains of Montana. I regularly travel roads to elevations of 5 to 7 thousand feet. I consistently get ~23+ mpg (calculated) in my 2012 JK 2 Door, with 3.21s and 5 speed auto transmission. My stick 2000 4.0L TJ had 3.73s and I was lucky to get ~19 mpg. I don't miss the lower gears at all.
 

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Too be clear, the new grand cherokee comes with 30" tires. :whistling:
Have you done the math? They are the same overall ratio within 1mph. Insignificant, although the Wrangler chassis isn't rated for the tow rating for other reasons. Just sayin'.:angel:
 

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Discussion Starter #10
highway only mileage is not realistic, though. you need to factor in stop and go, city, and hills. 3.21s will be worse off below highway speeds and through hills.
I never said it was a scientific test, nor did I indicate that it covered all variables; to the contrary, I was very clear about the limited distance traveled, speed, and environment. I probably log half of my miles on highways and interstates and hills aren't a factor for me, I live in Louisiana where the highest point in the entire state is only 365' above sea level. I said the 3.21s work fine for me in my smaller, lighter, more nimble JK. So many people on here BASH the 3.21 that I wanted to show that for some of us that particular set-up works just fine. I promise you that if it was as junky or ill-concieved as many of you imply that Jeep would have never offered it....why would they if the automotive press got the keys to a bone stock JK and slammed it for it's poor performance? Makes no sense. May of us (myself included) drove older non-JK platform Jeeps for years and are THRILLED at the spirited performance the 3.6L/Auto/3.21 provides. Ever driven a 4.0L TJ with 31s' pushed by a 3.06? It's a lead-sled in comparison to my JK which will get up and scoot...actually a good bit better than my wife's Mazda turbo-charged 4 banger in her CX-7, and most of the automotive world have considered it a fairly "sporty" compact SUV.

I'm sorry for my "rant" but it get's a little old hearing about the percieved "limitations" of the 3.21. Jeep builds it for a reason; might not be the right gear for some of you macho guys, but some of us older, more "seasoned" individuals like it just fine.
 

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I never said it was a scientific test, nor did I indicate that it covered all variables; to the contrary, I was very clear about the limited distance traveled, speed, and environment. I probably log half of my miles on highways and interstates and hills aren't a factor for me, I live in Louisiana where the highest point in the entire state is only 365' above sea level. I said the 3.21s work fine for me in my smaller, lighter, more nimble JK. So many people on here BASH the 3.21 that I wanted to show that for some of us that particular set-up works just fine. I promise you that if it was as junky or ill-concieved as many of you imply that Jeep would have never offered it....why would they if the automotive press got the keys to a bone stock JK and slammed it for it's poor performance? Makes no sense. May of us (myself included) drove older non-JK platform Jeeps for years and are THRILLED at the spirited performance the 3.6L/Auto/3.21 provides. Ever driven a 4.0L TJ with 31s' pushed by a 3.06? It's a lead-sled in comparison to my JK which will get up and scoot...actually a good bit better than my wife's Mazda turbo-charged 4 banger in her CX-7, and most of the automotive world have considered it a fairly "sporty" compact SUV.

I'm sorry for my "rant" but it get's a little old hearing about the percieved "limitations" of the 3.21. Jeep builds it for a reason; might not be the right gear for some of you macho guys, but some of us older, more "seasoned" individuals like it just fine.
I understand you're happy with 3.21s, they are a fine gear ratio for most people. No intention to offend anyone. In fact, the 3.21s are only considered bad for pre-2012 models, or for people who want to up their tire size.

I was just pointing out that your fuel economy comparison doesn't really have any merit. the dollar savings you mentioned wouldn't work out in the real world. 65mph in 4th will get worse mileage on a flat highway than in 5th gear, you have quantified that. same as 3.73s or 4.10s will get worse mileage at 65mph than 3.21s. but, you cannot simply draw dollar savings signs based on those figures alone, because they are pretty meaningless in the real world.

3.21s were put in the jeep to up that highway gas mileage figure to the best possible number (probably about 0.5mpg higher), but that doesn't help the city mileage. I'm sure that jeep weren't foolish enough to give out 3.21s to the auto reviewers. it's not like they hand out base model sports either.

And just to throw a number out there, my heavier JKU with 3.73s has got 20.5mpg combined over 11,000 miles.
 

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I was just pointing out that your fuel economy comparison doesn't really have any merit. the dollar savings you mentioned wouldn't work out in the real world. 65mph in 4th will get worse mileage on a flat highway than in 5th gear, you have quantified that. same as 3.73s or 4.10s will get worse mileage at 65mph than 3.21s. but, you cannot simply draw dollar savings signs based on those figures alone, because they are pretty meaningless in the real world. You are absolutely correct. I am sorry that I wasted your time publishing my little impromptu test this morning. I am stupid and incorrectly assumed that it was OK to spend my time publishing my findings and that some folks might like to see it. Please forgive me.

3.21s were put in the jeep to up that highway gas mileage figure to the best possible number (probably about 0.5mpg higher), but that doesn't help the city mileage. Why the heck did they put 3.06s in Saharas way back in 2000? Fuel economy? Couldn't be....'cause mine NEVER saw 15MPG.

I'm sure that jeep weren't foolish enough to give out 3.21s to the auto reviewers. it's not like they hand out base model sports either. Wrong. They tossed the keys of a 2007 bone stock (steel wheel "as cheap as they build 'em") JK to the editor of a very recognized off-road magazine and sent him to Gilmer, Texas to get his un-biased opinion. He was impressed at the (then) new Wrangler.
sorry:surrender:
 

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Ya all crack me up, still going on about gear ratios.

Lower gears give better mileage at stock tire size. A higher gear (numerically lower) will give the same results with a larger sized tire, with a slight reduction due to various forces such as friction, weight, rolling resistance, etc.

I run 3.07s with 33s and a 4.0... I know it's not a JK. I pulled 18.25 mpg on a 190 mile trip to Rausch Creek through the mountains of Pa. I drove it smart, light gas, kept the rpms around 2600 (70 mph) I drove it hard on the way home, and pulled 15.5. Ran it between 2800 and 3k at around 75-80.

Drove half of the way with the mod on the forum InvertChaos. He has 33s and 4.56s, and we stayed together with no issue.

You guys need to learn that you can throw numbers around, but get out there and check out how the different engines, trans, gears, tires, and driving styles all play a part in each and every persons personal experiences. No two people will be alike. Do you guys understand that the gear ratio is important, but downshifting can effectually give the same final drive ratio. For example my In third my trans ratio is 1, tcase is 2.72:1. the axle gears are 3.07s. the tires were originally 29s. That gives me this cool number that can be changed into mph if you know the rpm. I'll do the calcs you need for 65 mphs. I will compare that with the number with 33s, and the number if I downshift to 2nd (1.54). I will also list the correct ratio, 4.10, to compare.

(stock) - 2314
(33s 3rd gear) - 2033
(33s and 2nd) - 3180
(33s,4.10s,3rd) - 2716
(33s,3.73s, 3rd) - 2281
Now, this isn't a very good example, but this is what I do commonly on very steep highway hills to pass. As you can see, the change seen in four inches of tire makes a very small impact on the rpms. In fact, it may be a preferable change. This doesn't take into consideration the torque converter, which makes a big difference. It also doesn't consider the increased weight and rolling resistance, or effects on the aerodynamics. However, this shows that 3.73s would actually give me a similar rpm as the stock gears did.

Currently, I downshift to pass on the highway. Let me tell you, the 4.0 wakes up at 3.5k.

Just think about these numbers and how much or how little gear ratio actually affects your final drive, when compared to your trans ratio which is selectable in increments for many of us.

I am glad the OP posted up what he found. He admitted the faults in his post right away. I don't think we can have an issue with that. He's right. lower rpms typically generate higher mpgs as long as your not lugging the engine or out of the powerband.

The point is, don't worry about the gear ratio, it doesn't even matter as much as some would have you believe :rolleyes: and just enjoy your rig.

End rant :thumb:
 
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