My previous truck was an '08 F150 S'crew, 5.4L with 3.55 rear end. It would consistantly get 15ish in city driving, 10-13 towing, and 20+ steady speed on the highway, depending on the speed. One highway trip I got 24.5 consecutively on 4 tankfuls with cruise set at 102km/h. That was a once in a ownership experience.

**Disclaimer**I am Canadian, the figures I've posted are hand calculated using the Canadian Gallon, which is 4.54 liters, vs 3.87 liters in the US gallon.