I average 18-22 MPG around town and on the highway with my '97 2.5l 4 cylinder, with stock gearing/suspension and 31's. I think the lowest MPG tank I got was going 85mph for 300 miles. I got 16-17 mpg.
If you're getting 15.5 MPG, there's something wrong with either your Jeep or your driving style.
Give it a tune up and possibly an oil change (use synthetic oil), before the trip. You'll see an improvement for sure
The easiest way to improve your driving style is pretend like your brakes are really bad and need replacement when ever you drive anywhere. That way, you dont habitually accelerate up to speed and then slow down with braking power, essentially useing your gas to heat up your brake pads instead of useing it to move forward and maintain momentum. 90% of the time, when someone accelerates hard out of a stop light, I see them brake hard again at the next light infront of me. 10-20 seconds later, I roll up next to him and lightly tap my brakes to keep from rolling into the intersection, having coasted off most of my speed in neutral prior to getting to the red light. Who used more gas to accomplish the same thing?