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Unread 08-30-2014, 02:36 PM   #1
jeepwrnglr92
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Stock JK gearing 3:21 vs 3:73 max tow option

I am sure this has been discussed but I am not really up on my gears. I ordered my Jeep yesterday and I only have a few days to change my mind on any options. So I could use some constructive feed back from people with 3:21 and 3:73 gears.

1. My 2013 JK 2 door has 3:21 and gets 20MPG average. Tows what 1200 or 1500 LB.
2. The dealer said the 3:21 and the 3:73 in the JKU gets the same millage. Hows can this be?
Programming in the trans?
No extra coolers or anything else added. I have a trailer hitch already but it comes as part of the package. So I am only really getting gears for $660 from the factory.

I rented a JKU for 2 weeks and got 20 average but I am sure it had 3:21 being a rental.

3. The dealer said the front gears are 3:21even with a 3:73 rear end. I thought you had to have matching gears front and back.

4. I don't tow much but want the ability to pull a 5500 lb boat 1/4 mile from the ramp one or twice a year for service. I know the max tow is rated for 3500 but i don't think a 1/4 mile will hurt it one or twice a year.

5 Can I pull the boat with a 3:21 rear?


Anyone have actual real world mpg numbers on the 3:21 and 3:73 stock Jeeps?
Please don't tell me to search. I did and I am still confused. Its a 660$ option so I want to be sure if I need it fine. If not Ill kill it.

I want good MPG as this is a street daily driver for me. Maybe some day more than that but downsizing to 1 car for a while. I drive 60+ miles a day so mpg is important to me. And Yes I know its a brick with an engine but I happy with my 20 to 21 on the 2 door now.

Thank oyu all in advance for your help on this.

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Unread 08-30-2014, 02:46 PM   #2
222Doc
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gears must match that dealer should know better. Unless you need tow more or going to larger tires might as well stay with what you have. Our 4.1 rubi when stock got 19mpg at 70.
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Unread 08-30-2014, 05:10 PM   #3
jeepwrnglr92
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so you are saying stay with 3:21? thats what my 2013 has or are you saying go with 3:73 for the 2015?
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Unread 08-30-2014, 05:16 PM   #4
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Go with 3.73 or 4.10 if you can!!!
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Unread 08-30-2014, 09:02 PM   #5
pentastarguy
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you are not going to 5500 lbs legally with any two door wrangler. 4 door max tow is 3000. lbs. "i believe. imho, you are looking at the wrong vehicle if you need to tow that weight.
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Unread 08-30-2014, 09:20 PM   #6
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1) OK, sounds reasonable.
2) Not likely unless tire & wheel sizes are also different or there is some other reduction in weight/drag. If 3.21 and 3.73 got the same exact mileage, one of them would not exist. With that said, the difference may be small enough to be negligible at the JKU curb weight.
3) Blatantly wrong. Front and rear ratios must match, otherwise something would tear itself apart in 4WD.
4) 5500 lbs is above the rated towing capacity, but it's your Jeep.
5) If it moves, I don't see why not. It may not be the wisest thing to do, but it should be physically possible.

Real world MPGs are defined by more than just gear ratios. Vehicle weight, tire size, and driving conditions like hills, traffic, and elevation all make considerable contributions to gas mileage, as does the actual fuel you burn (e.g., oxygenated/ethanol enhanced, vs. good old fashioned unadulterated dinosaur juice). JKs come in many configurations, and each is slightly different.

I'll offer you this information: I drive a 2012 JK Sport with stock 29.6 inch tires and 3.73 gears and I generally get about 19-21 MPG average mileage with a 40 mile one-way daily commute.
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Unread 08-30-2014, 09:59 PM   #7
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Gears have to match front and rear. The dealer is either stupid or blowing smoke up your ***.

You won't find anyone regretting having more gear, but everyone regrets 3.21. It shouldn't even be offered.

As for gas mileage, it is entirely possible that 3.21 ends up worse. The trans will shift more and not like to be in overdrive (or if a manual, you'll spend more time in 5th and not 6th).

3.73 is a no brainer. 4.10 is even better, but it would have to be a Rubicon
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Unread 08-31-2014, 06:02 AM   #8
jeepwrnglr92
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Thank you all for the info. I thought the dear was full of it . Figured front and back had to match but there was no mention on the build sheet of the front axle ratio. I am going to assume (hopefully correctly) the factory will match them if I ordered the 3:73 rear. Don't want to sound dumb here but I never understood the 3:73 number. I know it has to do with the ratio of the wheel turning to the drive shaft? I don't know how the Jeep can be rated for the same MPG with 3:21 and 3:73 gears. As for the configuration, stock 17 factory wheels and tires . Yes I know 5500 I over the tow weight so I may have to rent a truck for those times I need to pull the boat out. Towing is not the main concern but MPG is. I was just curious if it could get it off the ramp even in 4 low. Don't want to be a uboat commander. I dont plan to tow with the 2 door JK, trading that in. I know better than that. I see all kinds of stupid in FL with people towing with 2 door JK. So from what I am reading 3:21 is not the best even though I have good MPG. Thank you jdm45 for real world numbers. I can live with those. I do the same kind of commute. 60 round trip. Anyone else wants to chime in I appreciate it.
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Unread 08-31-2014, 06:40 AM   #9
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I'm kinda late to this convo but 3.21's are awful...I have 33.5" tires, regeared to 4.10's and my mileage went up about 1.5mpg. I'm now at about 17mpg. I do have the older 3.8L v6 not the new 3.6L v6.

Also, the ratio 3.21 3.73 4.10 etc is actually technically 3.21:1 3.73:1 4.10:1. That represents the number of times the driveshaft spins for each rotation of your tire. The higher the number, the "lower" your gear ratio is giving you greater mechanical advantage and more torque at the rear wheels.
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Unread 08-31-2014, 09:00 AM   #10
jeepwrnglr92
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Thank you for explaining that. I was told the engine would rev higher with the 3:73 thus lowering the MPG. I plan to keep stock on it for a while. When the factory tread wears out Ill put 32 on it. I think I can do that without any lift from what I have been reading but that like 40K miles down the road.
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Unread 08-31-2014, 09:32 AM   #11
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Quote:
Originally Posted by jeepwrnglr92
Thank you for explaining that. I was told the engine would rev higher with the 3:73 thus lowering the MPG. I plan to keep stock on it for a while. When the factory tread wears out Ill put 32 on it. I think I can do that without any lift from what I have been reading but that like 40K miles down the road.
Not sound logic. Engines have efficient operating ranges. It takes x every to move a vehicle at a certain speed. It takes y amount of fuel to create that energy. Adding a couple hundred rpm would have a very minimal impact (not perceivable in fuel economy). However (and I can tell you from first hand experience) that running numerically lower gearing can reduce fuel economy. More shifting of the transmission (that alone will hurt mileage) and not being able to run in overdrive at certain highway speeds. Downshifting more to accelerate, etc.

I run 4.88 gearing and push 37" mud tires on heavy beadlock wheels. I have endless mods that reduce aerodynamics. With all that, I still average 17.5 mpg on long trips (and usually from MI through the PA mountains). Prior to that, I had 35s and 3.73 gearing. Want to guess what my highway mpg was? 18 mpg. The gearing was really inefficient. A lot more shifting, 5th gear on the highway.

I would love to see someone do a back to back comparo of stocks JKs with 3.21 and 3.73 as daily drivers. My money would be on the 3.73 for better mpg.

Chrysler does it to please a computer model for CAFE standards. Unfortunately, the correlation to real world driving is very low.
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Unread 08-31-2014, 09:35 AM   #12
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I went from 3.21's to 4.10's and got a very nice increase in mpg. I'd assume 3.73's would get better economy than 3.21's but idk how they'd compare to 4.10's. That's what I'd like to see
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Unread 08-31-2014, 09:42 AM   #13
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You must have a standard tranny, because 4.10 gears are a joke when you have an auto--

Even with stock tires 31/32"--a waste of time and no pleasure, especially on the hiwy-

My mileage went from 15.2 mpg (33.6") tires to 17.2mpg/70mph, by regearing to 5.38s-

That opened up a completely different jeep, whole different operations-

Guys on this forum are sick of my ramblings--so

Good luck

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Unread 08-31-2014, 09:55 AM   #14
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Quote:
Originally Posted by JIMBOX
You must have a standard tranny, because 4.10 gears are a joke when you have an auto-- Even with stock tires 31/32"--a waste of time and no pleasure, especially on the hiwy- My mileage went from 15.2 mpg (33.6") tires to 17.2mpg/70mph, by regearing to 5.38s- That opened up a completely different jeep, whole different operations- Guys on this forum are sick of my ramblings--so Good luck JIMBO
I do have a standard tranny...gotta love a six speed
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Unread 08-31-2014, 10:54 AM   #15
222Doc
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Quote:
Originally Posted by jeepwrnglr92 View Post
so you are saying stay with 3:21? thats what my 2013 has or are you saying go with 3:73 for the 2015?
If you are buying another jeep always go with highest gear ratio offered for the model. 3.73 if not a rubicon and 4.1 if a auto rubicon- 2012 and up.

This way you are more set for larger tires. To have to gear a new one latter cost a lot and will not be covered by the factory warranty
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