20mpg is on the high end, best I've seen is around 21mpg and now average 15-18mpg with 36" tires and 5.38 axle gears. Your speeds are sustainable, but don't expect to pass other vehicles, and plan on down-shifting for the slightest grades. With my gearing and engine mods I can now hold 5th gear and often accelerate up highway grades that required me to downshift in the past.
The 2.5L is a good engine and a lot of fun after a few power mods. Just know what you're getting into and all will be well.
Check out
www.4bangerjp.com for a good 4cyl jeep forum.