Yeah I think the 82 is to low causing the engine to freak out a little cause I can cruise at 80 with a lift and 37" tires and get 19mpg! Bone stock I was well into the 20's on the highway. So I bet if you put "normal" gas ie 87 octane in it I bet your numbers go up. I would get 18-19mpg in the city stock.

Plus who gets differences of 5 miles per gallon on the highway? I drive on the highway a lot and my mpg's for a trip never very more then 1-2mpg.

 

Yes, what he said. lol. This whole "getting a more complete burn" thing with higher octane and "getting more horsepower and mpg's with higher octane" is just completely incorrect. Yes, higher octane fuel burns slower than low octane fuel. Fact. BUT, the reason one engine performs better with one octane level over the other is dependent on the particular engine's bore, stroke, compression ratio, and timing mainly, amongst a very long list of other things. To put it plainly- If your fuel/air mix burns for a longer time than your piston is able to compress......well...boom! There's no increase in performance about it, but rather a very deadening effect on your components over time. That's the kindergarten version of the story anyway.

 

Thank you. Speaking for those of us who are less mechanically inclined (but learning!) the kindergarten version is much appreciated.

 

I'm going through fuel mileage tests right now. I'm on my 3rd tank of "premium" and I've lost about 3 mpg while running it. After this tank I go to the mid range stuff. There is little to no change on how the motor performs. So far it is looking like the 87 does just fine, and unless some incredible change happens with the 89 octane I wont be changing the fuel the motor calls for.

I typically go through 2 takes of gas a week. Since 90% of my driving is my commute to work my driving habbits don't chagne day to day.

 

Haha no problem! I'm by no means a mechanical genius, but these few things about engines I do know. The reason a slower burning (higher octane) fuel "can" have potential to produce higher horsepower (in an engine designed for it, of course) is because the slower burning fuel burns for a longer period of time, thus producing a longer power stroke of the piston. i.e. race cars/boats burning 110+ octane. In those engines the high octane fuel/air mix burns "slowly" and for a longer time inside the cylinder thus creating more energy and downward force of the piston, which is turning the crank, thus creating horsepower. But, the only way the engine is getting that full energy potential is if, and only if, the valves are closed and the piston is able to move downward. Eventually, that piston reaches the end of its stroke, and the crank is forcing it back up again to expel the consumed fuel/air mix (exhaust). So, if your fuel/air mixture is still burning and hasn't been consumed and become exhaust at the point that the piston's power stroke is over and it's beginning its travel back upward, you're in trouble. The burn speed of your fuel must match your particular engine's timing, which is dependent on numerous things. Moral of the story....as far as I know (disclaimer ), JK's are designed for 87.