I have the stock (255/75/17) tires, no lift, hard top, AT, and 3.73 rear axle. I usually only get less than 17 MPG with mostly urban/suburban driving. (less than 50 MPH, lots of speed variations, etc.) Before I turned off O/D for urban/suburban driving, I was getting 15.4, now I'm at 16.8. (computer numbers)
Mixed driving (50-75% highway) at around 18 MPG. (calculated)
Almost pure highway driving is north of 20 MPG (calculated), as long as speeds stay below 65 MPH. Anything more than that and MPG drops like a rock.
Of course, MPG above 40 MPH is negatively affected by top/doors off, so I expect this spring (today included - it's gorgeous today!!!) to be worse than last fall. (I bought it at the end of September.)

 

What situations should I turn O/D off. When should I turn it off to increase gas millage. All I know about it is that it shifts to some gear when you're going over a certain speed (35 i think). How does turning it off help.

 

I've been turning it off when I know my speed won't go over 50 MPH and the speed could possibly vary.

I experienced today taking off the front top, plus all the doors, but leaving the rear top on, is kind of like driving with a parachute at highway speeds. I dropped off my daughter at a friend's house and took "the long way home" to enjoy the beautiful weather. What normally would have been 22 or 23 (computer) was showing at 18!

(CRAP! Just noticed my typo in the previous post!)

 

X2^^ this worked for me as well

 

All I know is I get terrific gas milage compared to my Dakota with a 4.7 I pull approx 10.8L-11.2L/100 highway and 13.9-14.5L/100 city.. or maybe the computer is crap?

 

Jesus that is good!

 

Wow your right! That means the liberty was absolute shit! Useing your calculator My suzuki SX4 gets 23 MPG.I thought it would be alot better than that.
I'm still pleased with the X though. As long as the gas mileage keeps improving I'm happy!

 

I average at a consistant 12.6mpg with my 33's ( usually run them if I know im not doing REALLY difficult wheelin). With the 35's I average about 15mpg. Doesnt sound like much of a difference, but it adds up with current gas prices. Either way, its worth it for the jeep!

 

Do you think this is pretty accurate?
http://www.fueleconomy.gov/feg/findacar.htm
What is normally used when gas mileage is posted IMP or US?
I apologize for my ignorance in advance!

 

Usually everyone in the States posts US MPG. Imperial is for us Canadians and UK people especially on the window Stickers of vehicles.

It's tough with the Canadian JK's as I am not sure if computer has a button to change over from L/100km to US MPG like my Dodge Magnum & my Dodge Ram CTD.