OK I will bite, I have a 2008 JK, 6SP, 35" BF Goodrich AT KOs, 3" lift, winch, winch bumper and rear tire carrier bumper. It is geared 4.88. See my profile for all of the mods done to my Jeep. I get 16 MPG highway, 13 city and commute 500 miles weekly to work. It turns 3000 RPM at 75 MPH and 2800 at 70 MPH that I have to run to keep in interstate traffic flow. It has a VIPER throttle body. I run the Hypertech REGULAR tune which gets me 1 to 1.5 MPG more than not running it. I do not have to do a lot of downshifting espically compared to when I ran 3.21 gears. I have run 4.56 which gave me 1.5 to 2 more MPG but less drivability and downshifting. I changed because of the power band and the downshifting driving TN hills. All of the loss from the MODS that others have said are right on specification...I can verify from experience. I do not see a problem with turning the 3000 RPM engine range as long as you keep good engine maintenence and run synthetic oil (i.e. AMSOIL). As a personal choice, I really like the power I get from the 4.88 gearing as compared to the 4.56 but note that the 4.56 will get you better gas mileage and the now famous forum "chart" is dead on the usage versus RPM range so study it carefully. I recently ran a 400 mile trip and noted that any MPH over 70 DOES NOT get you any gas mileage gains with 4.88 gears so let your intended USAGE (highway versus wheeling) be your ultimate guide.

 

I would like to follow up on my 500 miles of commuting daily in my 4.88 geared JK with 6 speed manual transmission. This means I top off my tank each day on my way home. I refill with around 6.25 to 6.75 gallons fuel daily according to how I drive those 100 miles. At the cost of $3.25 per gallon, this is around $20.32 daily I pay for fuel. In a 240 day work year I will spend $4875 in fuel to commute to work, not counting personal use (i.e. wheeling ). If I would have stayed with the 4.56 gears I would have saved a little money, $280 to $500+ yearly but lost some driveability. My goal now is to move to a good small car as to not wear out my JK and study the RIPP supercharger upgrade for the 3.8L engine. As you can see by my math I can justify the purchase of the RIPP supercharger in 3 to 4 years if I drive a car that averages 25 to 30 MPG, at least that is what I will tell the wife . My problem is that I LOVE driving my JK. I do not mean to steer away from the thread subject, I just want to convey that the DRIVEABILITY is what you should re-gear for...it is a JK, not built for good MPG.

 

That is the same set up I have 5:13 Yukon gears and 35/12.50/17 Goodyear MTR/K , Suoerchips MPG is the same as stock.

 

Go with the 5:13 and never look back your JEEP will thank you

 

Everything else the same except the tire size is 285/75/R17 (33.8")...should I still get 5.13 or 4.88?

 

Regarding the high RPM topic on the JK 3.8 engine, it is a very different engine than the previous model straight 6. The 3.8 thrives on a higher RPM and it won't hurt it. And for the poster who doubts the posted mpg's of 5.13's you have no basis to speak on it since you haven't done it. And I didn't see one single post were someone said they were running their RPMs at 3000. I'm going about 80 mph at 2500 RPMs on a level roadway and I don't care to be going over that speed at any long length of time anyway. At 3000 rpm I imagine I would be darn near 100 mph on the same level highway.

 

Um, dude on page 1 there's talk of a JK turning 2,800 to 3,000 RPM on the freeways.

And, can you show me some proof that the 3.8 "thrives" on a higher RPM? ALL gas engines make more power at a higher RPM...that doesn't mean that they are best to run at those RPMs all the time for longevity, or mileage.

No, I haven't regeared a JK yet. But I have regeared plenty of other Jeeps. I've run Jeeps with 3.07s, 3.55s, 4.11s, and 4.88s.

I'm not calling anyone here a liar. I'm just saying that I don't see how you could possibly be turning crazy RPMs on the interstates and freeways and getting good mileage. I'll bet that with lower RPMs, you'd lose a little "power" but you'd pick up mileage.

 

zmotorsports
I agree with the counter-cost of buying an economical car for commuting versus driving my 16 MPG JK, it is an even wash in the first year spending. The good thing is that I already have a VW Passat that I will put about $3000 in for this task...I know, more money I could be spending on that RIPP supercharger I want (need)!!! I am with you on having the gas saving money for more mods for the Jeeps!

WVman
I drive my 6 speed JK every day 100 miles minimum: 75% is at highway speed turning 2250 - 3000 RPM (55 - 75 MPH) and 25% is seconday road driving at 1800 - 2250 RPM (45 - 50 MPH) combined with stop-and-go driving. I am regeared at 4.88 for 12,000 miles. I see no adverse affects on the engine running 5W - 20 synthetic oil. This is of course my seat-of-the-pants feel for the engine and listening to it run, no scientific wear evaluation. This does not mean that I will not spin a journal bearing soon but all is good so far. Running 3000 RPM at highway speed does not seem to be a strain on this engine. I just did a 400 mile jaunt and it ran and felt very good. Now if this was my Cherokee with the 4.0L engine, I would be a little worried. It seems to me that I stressed the 3.8L more running the 35" tires and 3.21 gears where I constantly had to downshift and rev the engine to recover. I hope others will comment on this subject...

 

To the guys asking about the OP's set up, I have a 2011 JKU auto on 35s and 3" lift. Also have bumpers.

I do appreciate all the advice and info you guys have given me, and I dropped of my Jeep yesterday to have the 5.13s in it. I am worried about the 80MPH range vs. RPMs, but I have to wait and see.

To all those who are looking through this post and wondering the same thing I was, I will let you know what I HONESTLY think of the 5.13s as soon as I have it back.

 

I'm running 35" with an auto and 4.88 and do not find the transmission hunting between 3rd and 4th gear. There has been the occasion where a 6-7 degree grade on the highway will necessitate 3rd gear to gain the required power. This was only the case on a couple highways in Colorado at altitude.