OK, now I'm getting really pissed off with this fawking Jeep. So I've been trying to get some mileage and power back since putting on some 285/70R17 tires on the wifes 2008 JK Unlimited x (A/T 3.73) and keep getting stone walled. I've installed a Mopar CAI, Magnaflow 18" muffler, and Flashpaq. I am currently running it on the Mileage XS in hopes of seeing some improvement but it is still only getting about 12mpg (hand calculated). The Jeep only has 27K miles on it and doesn't get abused at all. The dealer just had it to do the tranny lines and I had them do a fuel system service on it just to see if it made any difference. When it was new we had no problem getting around 21mpg (hand calucated) with the stock 255/75R17 tires. Any suggestions or other issues I am not aware of? I can't imagine that the cats would be plugged at such a low mileage. I am at my wits end and the wife is getting pissed with the amount she is having to pay for fuel so any suggestions are welcome.

 

Is your speedo/odo accurate for the tires you are running?

 

what pressure are you running your tires at?

 

+1 on speedo cal. My 08 JK was reading 12-13MPG hand calcuated using the tripometer. However, due to the speedo being off, so was the trip/odo. Now, after using the AEV Procal, I get 17-18 in the city and some highway.

 

You bet.

30psi is what works best with her setup.



Just so we can get the basic responses out of the way - I have worked in the automotive aftermarket for 20yrs ( 9yrs performance and 11yrs offroad specific) so I know and have covered all of the basic stuff. This is the reason I am so frustrated with this POS at the moment. No matter how you slice it, allowing more air in and out of an engine will make it more efficient. Throwing some optimized electronic tuning at it only makes things better. I have yet to check the plugs or wires but there has got to be something wrong other than just winter fuel.

 

I don't know about the winter fuel....got to be good being it jumped .46 today alone here. For $3.94/gal for 87 I would hope it would be the purest, cleanest burning substance on the face if this earth! Some how I think I'm living in fantasy land wishing that! Some jackass farts in the middle east, a trash can gets set on fire at a refinery in Washington St and all hell breaks loose! Doesn't OPEC & Exxon understand we drive modded vehicles that suck on mpg's. Oh, wait, they do. Scratch that comment. Anyone want to jump on board & see if they will give us a group buy on gas????

 

Never had a lot of success with the Mileage XS tune from Superchips except for loss of power and I was always able to do as well or better with the regular 87 tune.

Not a whole lot of difference in tire size there, but make sure your speedometer agrees with the actual rate of travel. Try to calibrate the actual speed with a GPS.

Is it actually getting up to operating temperature as if the thermostat is opening early you will be continually running in open loop and sucking away fuel.

Turn overdrive off, better to be running higher in the rpm range than regularly downshifting at cruise speeds.

 

I had 2011 with SAME problem. I ended up regearing my 2011 JK with 4:56 gears and it was like a different Jeep. Got fuel mileage back up into the 16s . It is the ONLY WAY to fix the lack of power from a 202hp engine that is turning a .69 final drive. Then I traded for a 2012 with penstar motor, WOW! Anyway hope this helps.

 

Haven't owned a set of open block-tread tires like your Pro-comps, but have read enough MPG threads here and elsewhere to realize they have a lot of rolling resistance. Traction or MPG; no free lunch.

Might be interesting to know how close to warmed up that 3.8 gets during an Edmonton winter. When I lived there my rides sucked fuel like crazy until around March. I did not know about louvers in front of the rad at the time, and always wondered why the heater sucked.

Sounds like you have everything else correct unless an analyzer can tell you something is wrong with the engine.

At a gazillion bucks a tank for fuel, it would not take much to justify a second set of wheels and tires for commuting. My commuting choice for a JK would be the Michelin LTX or back to SRAs.

 

Thanks for all the input guys. OD is always off when in the city. Engine gets up to operating temps within 20-30 blocks at temps around freezing. We have had an abnormally warm winter this year with next to no snow and only a couple of weeks worth of really cold so there haven't been a lot of long warm up times. I just returned it to stock and then did an advanced tune and made sure to verify that the tire size was adjusted. One interesting note is that the tire size setting in the Flashpaq had reset itself to 30.25" and not the 32.75" that I previously programmed so there may be the possibility that it didn't actually save the changes I made when I did the original tuner download. I guess this could explain why the mileage has been sucking donkey balls for the last couple of months. I'm going to have her fill it up again and reset the average economy indicator and tripmeter and see what happens. I'll report back on this in a few more days.