You can't just partially fill your tank and judge gas mileage by that. The fuel gauge on TJ's is notoriously inaccurate, so the best way to calculate mileage is by filling up the tank and resetting the trip odometer. Drive around until it's low on gas then fill up the tank again. Look at your trip odometer, and divide the number of miles driven (could be anywhere from 180-250 mi., YMMV) by the amount of gallons it took to fill up the tank.
"Anything to do with the land, I love."
"So it's probably eighty percent luck and twenty percent skill."
I never could figure what the sky was thinking, but the soil, she don't keep too many secrets.