Stop asking so many questions!!
Love you.........

 

I know everyone is talking about about 4.88 in the 3.8 auto but I have a 3.6 with 3.73's and I average 15mpg city, 17.3 hwy. I'm thinking I would be fine with 4.88s. What do you guy's think?

 

I'm running 456 s with a 6 speed and lower end 1st~4th short shift range but 5~6 runs around 2500 at 65~70 on the freeway I'm running 35 s who cares about mpg I thought jeeps were about fpg if you want fuel economy buy a prius so my HID' s can blind you as I drive behind u

 

My daily driver is a 09 JKU Auto Rubicon. I added alot of extra weight, front and rear XRC bumpers, Warn Winch, MT Sidebiter wheels with 35x12.5x17 KM2 tires, additional steel tank and transfer case skid shields. RockKrawler 3.5" lift, Bilstein shocks.
I elected to install Yukon 4.88 gears. The charts and math just worked out. The KM2 tires are actually 34.25 inches in diameter. I used a Pro-Cal to reset the gear and tire size. I compared my trip mileage to my Tom Tom GPS, and my odometer is accurate to 1% on trips. Acceleration from a traffic light is fair, highway acceleration is good. (My opinion). Adequate for my needs.
I recently took a trip Phoenix-Tucson-Phoenix, approx 200 miles, on cruise control held 65 mph, gas mileage was 18.1.
Around town it's usually averages 14.1 mpg.

 

My 4dr has been my daily-driver for the past 63,000 miles -- the entire time with 35" Toyo's, AEV pintlers, heavy bumpers, sliders, winch, etc. and 4.88's calibrated with a Pro-Cal. A typical day for me is a 30 mile one-way commute to the office, about 2/3rds of it at 70 mph and the rest in stoplight-to-stoplight traffic -- I averaged about 14.5 mpg. When I switched to Raceline Monster Beadlocks my average dropped to 12.5 mpg. I figure the loss is a combination of the extra width and the added 10+ pounds, per wheel, of rotating weight.

I do live in the Dallas area (ie, no hills and 500' above sea level).

 

This may help decide -

http://www.grimmjeeper.com/gears.html

 

I'm running 37" Toyos (really heavy tires) in my auto 4 door with the 5.13's and Super chips 87 tune plus all the heavy mods in my sig & tools in back and I average 14 mpg's on the highway. When I had Toyos in a 35", I was getting around 16 mpg. This was at 70+ mph at 2200 RPMs in hilly New England. I say go with the 5.13's as most suggest.

 

K so after much much much much contemplation, I have decided to go with the 5.13s. I'm still a little worried that 80 MPH will be ridiculous, but the shop reassures me that this is better for my Jeep over the 4.88s. Just wish I could have driven JKUs with both ratios to help the decision.

 

I don't have 35s. I realize that 33s to 35s is a big difference.

Having said that, my tire/wheel combo weighs 76 lbs. per corner so not lightweight by any means.

I just did a little experiment where I kept my OD off all the time through two tanks of fuel to see what affect running 2,500 RPMs on the highway would have on gas mileage.

Answer: It got worse. Using OD, I'm posting 15.5 MPG average with basically every tank. With OD off, turning more RPM on the highway, I saw 13.2 MPG first tank, then babied the second tank to a 15.1 MPG finish.

I fail to see how on earth you guys are getting better MPGs on the highway by turning ridiculously high RPMs in the 2,500 to 3,000 RPM range. It just doesn't make sense.

Also, until post #28 this thread was full of fail. The OP asked about gas mileage with 4.88s. He did not ask about power. Everybody knows that if you just want power, throw in the deepest gears they make for your axles and be done with it. He's specifically trying to find a good balance between driveability and efficiency... and personally I think 4.88s might be great.

Anyway, I'm very interested in this topic too because I'm looking at gearing myself.

 

Hell, just this week I was reading about an old vintage Jeep model that is highly sought after these days, and one of the things listed to "watch out for" is that since those models were geared so low, they turned a lot of RPM on the highway. Therefore, if the one you are looking at was driven on the highway a lot, it could experience early engine failure.

I mean, I think we all know that a V6 singing at 2,800 RPMs for hours at a time is not ideal. For the power and driveability it will make us JK owners happy. But for longevity, and for fuel economy which is what this thread is specifically about, I just don't see it.

I've seen two maybe three posts in this entire thread from 4.88 owners about what their experience with MPGs has been. How about some more.. I'm very curious. I don't want to drop $500 and a Saturday morning on regearing only to find out that my MPGs don't respond.