Better MPG in a Jeep Liberty.?

The jeep is a 6 cyl, 3.7 L, Automatic 4-spd, rear wheel drive. I have done what I normally recommend without any success. It's estimated mpg is supposto be 18-21 I am only getting about 14 in city and 18 highway. What could be causing this it's at 55,000 miles. I bought it used everything checked out on carfax but I don't know it's past matinance. I installed a K&N air filter with no increase in mpg.