tpm152

My speeds when averaging 22 mpg fully loaded up, climbing some extended grades, and having the AC on full blast average between 70 mph (in PA where the speed limit is 65) and 75 mph (in WV where the speed limit goes up to 70). So take that as you will but I would hope people weren't averaging 80 - 90 mph in a JK. That seems a bit suicidal, even at stock height I feel uncomfortable going 80+ the few times I've hit those speeds to pass trucks

Also the engine speed being described is the reason I love the 6-speed. Going 70 mph my engine is turning almost exactly 2000 RPM in 6th gear. It has plenty of torque/power to maintain speed on any grades except for 1 or 2 on the entire stretch of I-80 through PA. On those one or two hills I just put it into 5th gear, the RPMs come up to ~2300 RPMs, giving me just enough of a boost in power and torque to tackle any hill. Plus then when I throw it back down into 6th you can barely hear the engine when you aren't climbing grades and the MPG is higher than if it was turning 2300+.

This is only for the 6-speed of course, the automatic in the Jk seems like a completely different animal.

matt852

yeah after 75 mph it just feels unsafe even at stock height. So for the JK engine to push the Jeep efficiently it should be at 2500 rpm when cruising on a flat free way?
How about when cruising on a street at 40? should I be at 2100 rpm or at 1800 rpm?

tpm152

The cut-off I place on lowest RPMs I allow my engine to see is ~900 at conditions other than idle. When I cruise around town at 35 - 40 if I don't foresee any need for acceleration I usually leave it in 6th and the engine is down around 1100 - 1200. It actually runs like a champ down that low and I regularly get 17 - 18 mpg when doing PURE city driving. So as for efficient engine speeds, the most efficient engine speed is the lowest possible engine speed for any one speed. This is why I like my 3.21 gears so much, 17 - 18 in the city and 22+ on the highway is great when you get to drive 425 miles plus on the highway on a tank of gas. Especially with gas creeping up past $3.25 in my local area lately

MuleShoer

When my JKU, auotmatic, w 4:10 gears, was stock it would see about 21mpg on my daily commute of 94 miles RT. The drive had 68 miles of rolling interstate and 16 miles of country roads. fast forward to now. When the JKU has 285 x17 duratracs, arbs bumpers, winch and a roof rack my MPG went down to 16.5. Plus the huge loss of power..Last month I regeared to 4:88's, got the power back but MPG stayed the same.
Last weekend I pulled of the rack now I am seeing 18.8mpg on winter blend.
I am also driving it for economy ie 70mph on th down hills and flats then I let it pull down on the hills to mid 60's... There is a point where aero dynamics trumps everything else.

snopro269

How many people on this thread have calibrated their speedos and actually take a old school notebook and data log the mileage driven and gallons used? I hate to tell people that the crappy MPG computer is not accurate. Mine says 18-20 MPG but in real calculated world that translates to 16 on the Highway at 60 MPH on two lane roads at 2000 ft elevation or Less and 14 MPG in city. 35 MTRs, Auto, with 4.10s

redrunner

So true. Here is some thoughts I have had (posted previously).

http://home.comcast.net/~kingslea1/Jeep/Gearing.pdf

They are aimed toward auto trans, but the concepts can easily be extended to manual trans. What is so apparent, though the subject is subjective and linked to specific vehicle setup, terrain, etc.; I really think the designers totally missed the mark on the auto drivetrain gearing. And I think the biggest issue lies in the trans gear selection. Now I understand, to keep costs down, they used an "off the shelf" tranny and didn't design a specific trans like GM did in 93 for the Northstar. But what they wound up with not only kills performance, I think it does the same for fuel economy.

jfbullseye

I Have a 2.5" lift 35 "tires and 4.88 gears . Am getting 15 to 16mpg . I just got my flashpaq last night . Will inst today and let you know how it goes.

Dronac

Any suggestions for a ratio to go with my 33's and an auto transmission? 4.88 or 5.13 for mpg?

Bodo

I just recently put 35x12.50 BFG KM2's on my 2dr. 6 speed Rubi with 4.10's. I haven't checked the mileage the old fashion way yet but what I saw with the onboard computer is that my mileage dropped from 19.8 to 17. Several days later I received the Superchips Flashpaq I had ordered to correct my tire size (and hopefully gain a little power). I programmed the tire size at 34.5" (at least that what my measurements were) and used the 87 octane programming level. My speedo is now close when compared with gps and the computer is showing 19.3 avg. mpg. Like I said, I haven't filled up and checked the mileage yet but will here shortly. As far as power I can tell a little difference. On a couple of hills on my way to work I had to downshift from 6th to 5th before the flashpaq. Now I stay in 6th but the Jeep does slow down some. Just not enough to go back to 5th.
All in all I'm glad I went with 35's over 33's.

jfbullseye

If your going to stay with 33s I would think that 4.56 will be fine.