Winter blend the each of the past 2 years has taken me from ~275 miles/tank to ~250.

 

Yeah, with many gas stations these days (here in Australia anyway) only offering the 10% ethanol blend in the regular unleaded, many people I know, driving all types of vehicles, have started complaining about a sudden drop in gas mileage. I did an experiment with my last vehicle (before I bought the JK), a Ford 6cyl Auto sedan which I've owned for years so I know exactly how much fuel it uses, and can notice any change in performance immediately (harder with a new vehicle you're still getting used to).

With the 10% ethanol blend, there was an immediate drop in power and torque, the engine ran rougher and with with a coarser note, it idled very roughly, and would ping heavily with preignition under even light loads until the ECU stepped in and retarded the timing all the way back. Instead of getting my usual 500-530km out of a 68Litre tank (city commuting), it was suddenly down to around 380-400km without any change in driving habbits, so a huge drop in milage was seen. I swiched alternate fills several times between standard and blended unleaded, and the results were always duplicated exactly regarding the changes seen using the Ethanol blend.

My mechanic told me that the changes will always be nore noticible on older vehicles, due to engine wear and also the more primitive and less adaptable ECU systems (particularly those like my old Ford that don't detect the fuel's octane and react accordingly but simply retard timing significantly when preignition is sensed, seriously curtailing power output on the spot and causing serious scares if you're pulling out in front of traffic or trying to overtake up a hill and suddenly find your power cut to nothing).

Since new, I've always filled my JK with Premium unleaded that doesn't contain ethanol, and even though it's more expensive I should come out ahead due to better milage and less maintainance (ethanol's known to rot fuel systems and kill injectors, particularly where the blend exceeds 10% unless the vehicle is E85 certified). I might do the Ethanol blend test on the JK now I'm getting used to it's driving characteristics and milage, and post my findings.

I agree with others here though that the computer's not always accurate (particularly over short sample periods) so I just reset my odo trip-meter when I fill the tank & calculate milage at the next fill to get an average.

 

I have only had my jeep 3 months(bought it new oct 31st) so It has had only winter driving on it and it dropped in those months...hope it gets better in the summer!!

 

I bought mine last summer when you could only get 10% ethanol gas, and I was getting around 16-17mpg in mixed driving, hand calculated. Starting in late fall, I've dropped all the way down to 13-15mpg. I thought maybe it was the AFE catback exhaust I added, so I switched back to my stock exhaust, and it didn't help any. I also keep tabs on my tire pressures so I know thats not a problem. My theory is that these engines really suck down the fuel when idleing. I do do a lot more of it in the winter months. Between that and the winter blended fuel, I think thats the reason behind my mileage drop. I'm gonna wait till this summer to really get worried if it doesn't change any.

Another thing just to think about too, maybe you wanna check for a cracked exhaust manifold. They are pretty common on the JK's, and I would think that if your getting any air in through the crack, that will cause the O2 sensors to tell the ecu that the engine is running lean, which will intern cause it to add even more gas.

 

my 08 2 rubi is about exactly the same. I blame winter mix.