I searched and searched but could not find a thread that just has some basic info about fuel economy, so here's mine.

I have a 2011 JEEP JKU with a 3.5" lift running on new 35" Duratracs with about 63k miles on it. Recently it seemed my fuel economy was getting bad. I never really paid much attention to it, once in a while glancing at the fuel eco gauge on the dash or noticing the 'DTE' after filling up. So I took the Jeep into a trusted shop to have the plugs changed and give the Jeep a once over. The mechanic replaced the original plugs with new OEM plugs. On the way home from the shop, I watched the fuel eco guage and right away it seemed like an improvement, getting better than 20MPG on a flat highway in cool and dry weather. Then, just a few days later it has worsened. I just now calculated my fuel economy the old fashioned way, using the odometer and gallons used. For highway and street use combined in clear weather, my fuel economy is 13 MPG. I pretty much drive like an old lady with the 3.8, but it does run well, no obvious power issues, the automatic tranny is doing well and so on.

In the past I seemed to recall seeing that 'DTE show something around 330 miles. Tonight when I filled up, it showed 280 miles?

With all that said, I wonder what your fuel economy / MPG is and/or if anyone else has experienced a similar issue with new plugs suddenly getting worse (or whatever else it might be)?

 

When I had my 3.8 in the mileage always varied especially heading into wind. I just usually weather vaned a semi truck on the highway to get good mileage because at 110kph I could never go faster. But my mileage always was about 20mp(imperial)g highway and about 18 around town. Now with the V8 I get 27/21 respectively. So with the 3.8 especialy, it is all about how hard the motor has to work. Now mine was a 6 speed manual so always had clutch lockup so wondering if yours is unlocking under load. Also mine is a Rubicon so had the 4.10 gears so the engine worked less. Plus the 35" tires take their toll on gas consumption.

 

In my 2012 Jku manual trans with 3.73 I tracked every single fill up over the course of 105,000 miles. I had a very detailed excel file and posted many times on here about what I was learning. My recollection is the only thing that was statistically significant was getting better MPG in warmer months. I tracked the use of different oils, the use of premium vs regular gas, change of air filters, etc. None of that made a difference.

 

I don't check the mileage much but I usually average 17 to 18.

 

That's quite revealing, thanks for the info.

It is apparent to me that the change occurred because of the spark plug change, but I would have expected a loss in power or some other sympton?

 

Yes, that's about what I was getting, much appreciated.

 

I often tell people that driving a lifted Jeep is akin to driving a large shoebox, LOL. Mine is a rubicon also, not sure about the auto trans unlocking but I'll check it out. Thanks for the rpely.

 

In the event anyone has a similar issues as mine and happens across this thread in the future, I'm posting some additional information.

In summary, after having the plugs replaced in my 2011 JKUR, the MPG on the highway dropped to 13MPG. Then, two days ago I bought a single bottle of Redline concentrate fuel additive and used it per the instructions. The miles per gallon has resumed to the average that it was prior to the spark plug change, maybe a bit better. I made a 40 mile trip along a highway with a few sloping hills in clear weather and the average was 17.1 MPG.

While I wouldn't know for sure, I'm supposing that some gunk got into one or more fuel injector(s) during the plug change which was cleared by the additive. The other thing I learned is if it ain't broke, leave it alone.

 

Wow, you guys are getting really good mileage. My Jeep weight about 6300 lbs, 37” tires and 5.29 gears. At an elevation of 5300 feet and normal driving I get about 12mpg. Oh, and it’s a 2015 automatic.

I don’t complain though. I bought it as a daily driver and built it into a toy. It’s not an economy car. When my engine dies I’ll likely put a 5.7 hemi and an 8 speed auto in it. If I get mid teens for mileage after that I’ll be happy.

 

If you are worried about gas mielage suggest you buy a Honda Civic or similar. Like I said earlier, even headwinds can severly affect consumption from one day to the next.