Best JKU gear ratio

    I've been giving some thought to buying a JKU. I'd target the second-generation engine/tranny (3.6L) in the 2013-17 age range, and I'd likely have 33" tires on it. I'm looking for guidance. A key consideration is fuel economy, and I want to get it right the first time. Unlike my current rock-capable 1998 TJ with the bullet-proof 4.0L engine, 3-speed auto tranny, I want a JKU that's much more highway-friendly and easier at the gas pump. Yes, I know, a Wrangler is basically a brick on wheels, but work with me here. Naturally, the JKU will still be expected to have some off-road capability, I'm just not looking for extreme.


    So, per charts I've seen on the internet, do I want to search for a JKU with either a 3.73 or 3.91 gear ratio? Or is there more to it? Other considerations on buying a used JKU? Thank you.

    When engaged in conversation with a fool, be sure he is not similarly engaged.
    Chief, DEJA Grammar Police

    3.73 is a great gear range all around. I ran that for a long time on my CJ and had no trouble at highway speeds with 33s. If you aren't running up and down the highway a lot you might consider going lower. I'm not familiar with where the 3.6 L power band is. Does it have low end torque line the in line 6 or does the torque come on at a higher rpm?


    Sent from my Pixel 2 XL using Tapatalk

    Member since 2000

    Jeeps Owned'79 CJ5:

    Specs-

    - 258 I6

    - '93 Cherokee fuel injection and HO head,

    - bored 30 over

    - with 4x4 cam

    T18 tranny with granny low / Dana 18 t-case

    Dana 44 front and back from late '70s Wagoneer, both locked and loaded with 4.88 gears

    SOA on modified YJ springs with shackle reversa

    Fiberglass tub with 6 point cage mounted to the frame


    2015 Grand Cherokee Limited
    Spec-

    Quadratec Floor mats

    Good point about "running up and down the highway." I'd like to be able to take long trips (think Florida, New England, Moab, etc.) with the JKU and not get too beat up at the gas pump. But, its primary use would be in and around Delaware. Right now, my F-150 is my "Fishing Command Post" vehicle, but I can see switching to a JKU for that function.

    When engaged in conversation with a fool, be sure he is not similarly engaged.
    Chief, DEJA Grammar Police

    I'm not sure what's available for them but is there an OD available or an extra low range tcase modification available?


    Sent from my Pixel 2 XL using Tapatalk

    Member since 2000

    Jeeps Owned'79 CJ5:

    Specs-

    - 258 I6

    - '93 Cherokee fuel injection and HO head,

    - bored 30 over

    - with 4x4 cam

    T18 tranny with granny low / Dana 18 t-case

    Dana 44 front and back from late '70s Wagoneer, both locked and loaded with 4.88 gears

    SOA on modified YJ springs with shackle reversa

    Fiberglass tub with 6 point cage mounted to the frame


    2015 Grand Cherokee Limited
    Spec-

    Quadratec Floor mats

    The factory tire size is very close to a 32. So a step to 33 would be very minimal. With that said to my knowledge the non-rubicon jk's came with 3.21, or 3.73 gearing. For this purpose I would be on the look out for a 3.73 geared Automatic JKU. Also keep in mind 35's on a jk seem to be the equivalent to 33's on a TJ.

    For reference, a stock JK with the standard 3.21 gearing will turn 1958 rpm's at 70 MPH


    A stock JK with the optional 3.73 gearing will turn 2275 RPM's at 70 MPH


    A JK on 35's and the optional 3.73 gear will turn 2080 RPM's at 70 MPH


    A JK on 33's with 3.73 gearing will turn 2207 RPM's at 70 MPH

    just for a frame of reference.
    I run the last configuration from the list above (A JK on 33's with 3.73 gearing will turn 2207 RPM's at 70 MPH)
    and find it to be a great overall combo. I've put a lot of miles on it including pulling trailers / boats and loaded with bunches of gear (surfing, hunting, fishing) and people and it's been fine. I don't think that I would like 3.73 & 35's because it would be sluggish to me. 3.21 & 35's would be unbearable. Again, just an opinion because I'm sure that people do it every day w/o issue.

    I went with 4.88s in my CJ because, at the time, I had a Saturn OD set up which brought my gearing down to 3.6? or something. I thought it was perfect for 33s. Then the OD done blowed up on me. 4.88s are fun on local roads. I keep it off Rt1 and I95 though.


    Sent from my Pixel 2 XL using Tapatalk

    Member since 2000

    Jeeps Owned'79 CJ5:

    Specs-

    - 258 I6

    - '93 Cherokee fuel injection and HO head,

    - bored 30 over

    - with 4x4 cam

    T18 tranny with granny low / Dana 18 t-case

    Dana 44 front and back from late '70s Wagoneer, both locked and loaded with 4.88 gears

    SOA on modified YJ springs with shackle reversa

    Fiberglass tub with 6 point cage mounted to the frame


    2015 Grand Cherokee Limited
    Spec-

    Quadratec Floor mats

    Thanks all three of you for the excellent info. Andy, now I'll reveal my ignorance....why wouldn't I want the 3.21 gearing and achieve the 1958 (lowest) RPMs possible? When it comes to fuel economy, isn't lower (within reason) better?

    When engaged in conversation with a fool, be sure he is not similarly engaged.
    Chief, DEJA Grammar Police

    Do you want fuel economy or off road performance?


    THAT is the question....


    Sent from my Pixel 2 XL using Tapatalk

    Member since 2000

    Jeeps Owned'79 CJ5:

    Specs-

    - 258 I6

    - '93 Cherokee fuel injection and HO head,

    - bored 30 over

    - with 4x4 cam

    T18 tranny with granny low / Dana 18 t-case

    Dana 44 front and back from late '70s Wagoneer, both locked and loaded with 4.88 gears

    SOA on modified YJ springs with shackle reversa

    Fiberglass tub with 6 point cage mounted to the frame


    2015 Grand Cherokee Limited
    Spec-

    Quadratec Floor mats

    I can tell you 3.21’s are no fun even with stock 32” tires. But you learn to live with it; just don’t expect to burn rubber, as the 3.6 doesn’t have ANY low RPM torque. But redline is 6400 or so, which means top speed is like 320 MPH or something :) I have 6 speed manual.

    Really what you run into is the power band of the engine. If you intend to stay on stock tires 3.21 gearing will yield the best MPG. But once you start uping the tire size the RPM of the engine drops lower and lower. Effectively dropping the engine out of it's efficient operating range. Truthfully a 33 inch tire will most likely not effect this at all. Your talking a difference 1958 and 1899 RPM's at 70mph.


    Now if the conversation shifts from hwy driving to offroad. You will see the benefits in every aspect, wether it be sand, rock, or hills.


    So a realistic estimate would be expect 1-2 MPG less with 33's and 3.73's then stock and 3.21. the trade off is better offroad ability and less stress on the vehicle in those situations.

    3.73 is a must 12 and up, if your lucky you can find a Rubicon with 4.10 but it will be rare to find

    as others have said the 3.73 will keep u kn the power band. If u go 3.21 the auto would be down shifting a lot or u would be down shifting the manual into direct. So on the highway u woulx actually be spinning the motor faster in direct

    Quote from astape

    as others have said the 3.73 will keep u kn the power band. If u go 3.21 the auto would be down shifting a lot or u would be down shifting the manual into direct. So on the highway u woulx actually be spinning the motor faster in direct

    Thanks!

    When engaged in conversation with a fool, be sure he is not similarly engaged.
    Chief, DEJA Grammar Police

    I have the 3.73’s and 34ish inch tires on the JKU. It averages around 16mpg mixed city highway and has done a best of 18mpg on straight flat highway. That’s with my wife driving who drives like a little old lady. The engine seems to like a little more rpm rather then lugging around like an old straight 6. Speed is a major factor too. Anything over 65-70mph and mpg drops off a cliff because of the aerodynamics.

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