My 86 cj7 4cyl with the stock tires would barely get 20mpg if driven easy and if I can remember was around 18mpg average. I went with a 3 inch lift and 35x12.50 tires with the stock 2.73 gears and my mileage really sucked(speedo read 40mph I was doing 70). I was lucky to get 15mpg driving it easy. I had to carry a 5 gallon gas can just to get enough range to drive to school. I went to 5.38 gears and that made the speedometer almost right. It read 5mph over at 55mph so it was pretty close but I had a bit higher rpms than stock. With that gear and tire combination I was back to 20mpg if I drove it under 55. Over 55 it really sucked with the 4spd trans, after swapping to a 5spd it didn't pick up any additional mileage, but at least I could drive 60mph without having to replace the motor every 5000 miles. Lowering the rpms isn't always good unless the motor is still in a range that it has good power and is able to hold your speed without a lot of throttle I guess.
With the bigger tires as long as I was real easy accelerating and drove slower than I did with the small tires the mileage was near stock. The biggest problem with the bigger tires is it exposes more tire to the wind and the weight makes accelerating take a lot more gas. So lower speed driving at a constant speed might gain mileage but anything else is worse for mileage at least in my case. Hopefully with the 6 cyl engine yours can handle the rpm drop and the weight of the tires and gain some mileage.
I have not driven that jeep in years and will probably sell it soon though. Anyone want to buy a slightly modified jeep
