woodweavil

Okay Im friggin lost. I have been allocated the funds to do a regear But now i must determine what to get I have a 2008 wrangler X (non rubicon) with the auto trans and the 24s package which gives me 3.73 gears I am running the 3.8 ltr v6. 18" sahara wheels with 35/12.50x18 mud grapplers. I live in Indiana and wheel here locally (badlands , silver lake, etc.) i am planning a trip to moab in the spring for the jamboree. this is what I need. I need to be able to run 60 to 70 on the hiway in overdrive without my trans jumping in and out of overdrive to maintain speed on slight grade changes. I would also like to get back my milage I was running about 18mpg when I went up in size my milage dropped to 11 mpg I knew it would drop but I had no idea it would drop that much. So that being said and ( BTW this is my daily driver I drive about 90 round trip daily.) What gear do I need to acheive this? I have been told 4.88 but from what Im reading that seems to not be the consensus! what the hell do I go with? i dont wanna be overgeared and I have no plans on going to 37s. I wheel a couple times a month if I can find the time. badlands mostly! and I will usally run the more difficult trails.:t hinking:

BlackNorthernJK

...but that's why the graph is intended to give you an idea, of course it doesn't take all variables into consideration. You'd have to publish a thousand different charts to cover all variables, it's just a ball park.
https://www.jk-forum.com/forums/jk-write-ups-39/gearing-82262/
the chart I compiled was researched heavily from a supporting vendor, and seriously the 'performance' interpretation coloured in portions truly are perceptual based, it'll help provide an idea and says it in the description, every chart is different, likewise everyones perception of performance is a little different...someone could sit down and try to compile all the different factors that will affect performance with each different gear, but there is WAY to much to take into consideration, least of which is individual driving habits

...sorry I'm at work and can't see your charts though, am curious.

JPop

OK, pretty close and what i would expect with the added performance of an exhaust and a programmer.

Now let's take that same scenario with the reigning champion of guys that start scratching their head as to which gear set is right with a set of newly installed 35s, 4.10s on their Rubicon and an automatic transmission.

So we know they need to make about 130hp to get up that hill/mountain and 3500 rpm gets it done. Their downshift out of OD is going to unlock the converter allowing a 10-12% slippage. So now they need 3900 rpm to make that climb at altitude. The downshift happens moving to the 1:1 third gear, rpms flash to around 3100, which doesn't present enough power to make it up the grade without a second downshift. The second downshift occurs, flashing the rpm range to upwards of 4800 with a 1.57:1 2nd gear which provides plenty of power but with good cause unnerves the driver as there is no relief to be found going back to third gear.

Furthering the issues associated with being under geared in this scenario is the locking and unlocking of the torque converter. This increases transmission fluid temperatures to a point where the PCM more readily engages TC lock up. With a locked converter and heated up transmission you need to be able to provide enough power to maintain your speed in 3rd or 4th. The only way your going to get a kick down to 2nd is at WOT.

While the numbers presented in this scenario would bear out that 5.13s would do the job with an automatic downshifted into 3rd, although there is no margin for error. That means, a head wind, temperature or humidity could part ways with the 5.13s from being a full time functional choice. Better to leave the slop factor in the throttle body and throttle blade as opposed to the PCM and transmission and run a marginally steeper gear (5.38s represent a less than 5% steeper gear than 5.13s).

In other words, don't fear the gear.

AZJeeper

Awesome linked thread! Great advice!

JPop

Hey, I thought this was just poking some fun at me the first time I saw the engineer reference but with a second time I need to clear the air.

I am not an engineer, that would be my father. I'm a technical sales rep in the professional photographic industry. However, I have done my share of messing around with gear ratios and automatic transmissions in the drag racing arena. The rules are of course totally different in a drag car than a jeep, but the math is all the same.

Also, I keep looking at these charts that get thrown around and not only do they operate under the premise of not having an OD gear, but they all seem to be using the 4.0L straight 6 as the power band by which they work from. The 3.8L is a totally different animal, with a flatter torque curve, and both peak hp and torque at 20% higher rpms than the 4.0. Foregoing the OD, also leaves a big hole with the differences between an automatic and manual transmission.

Anyway, I guess that I should make a couple of charts (1 6 speed and 1 automatic) with some multicolored cells so it paints a better picture and doesn't lead people astray. Unfortunately for some I don't think it will matter as they only seek affirmation from someone who agrees with them on whatever they set out to do.

JPop

Well, here's a preliminary one.

I think it's pretty good guideline and I tried to break things down a little differently than some of these other charts. I also did away with the red they used as I thought it had a negative connotation for what isn't a poor choice, but one that means a good choice for performance.

On the yellow fields and what I determined to be the bottom end of the gear/tire combinations I would note that these are not gear sets that you would want to upgrade to, but rather what you could get away with if you decided to upgrade your tire size.

Whenever you get to tire sizes of 35" and beyond it's best to look to the high side of the green fields or better. This is primarily because I believe the majority of people that take on the expense of 35s or better really do have more intent than just driving around the mall.

This certainly isn't a done deal nor by any means something that isn't open to criticism. I used the dyno information available to determine where enough torque existed to get a vehicle rolling and enough hp to sustain highway speeds.

All RPM numbers are at 70mph.

BlackNorthernJK

JPop...I'd have to say those charts look pretty good, nice work. Hopefully this helps clear some things up for people

REL1203

Nice work JPOP

JPop

Thanks!

It's a start and a work in progress. I definitely want to make the two charts more robust than the others I've seen and did some quick work on accounting for tire weight.

SilverJK12

So basically at 4.88 on 35s manual puts me in the blue. So I should be fine going to 37s in the upper green? correct?