I have a 2000 Expedition with the 5.4 liter 2 wheel drive. (K&N drop in filter / cat back flowmaster exhaust) I get 11-12 mpg in town and average 17-18 highway. The best I ever got highway was 19 mpg when I went from Vegas to Reno, I was driving 70 mph on cruise most of the way. I'm sure my wheels/tires hurt my mileage a little as I have 275/55/20's

You are not going to do much better. These things suck gas.