Fuel mileage can vary from one F150 to another with the same engine/transmission combo. Reason is, the rear axle ratio. I will only speak of Ford, as I don't keep up with Chevy specs. There are several gear ratios you can get in an F150, and 90% of customers will not ask about the gear ration, and well over 50% will just give me a blank stare if I get into gear ratios for fuel economy.
Here are the gear ratios available in the 09-2010 F150:
3.15------------best mpg, weakest for towing
3.73---------lowest mpg, best towing
I've been driving 2009 F150 STX Supercab for the last year 4.6L 3 valve V-8 with 3.55 gears and the new 6 speed auto. I commute 56 miles each day round trip, half city, half highway. I average 19.5 mpg combined. On a highway only trip, I've gotten 22 mpg @ 60mph, 21 mpg @ 70mph. Previously since 2004, I had driven F150 supercabs with 4.6L 2 valve engine, 4 speed auto and 3.55 gears, and I averaged 15.5-16.5 mpg average, with 18 mpg on long highway trips.
If anyone is interested in a Ford, let me know. I have sold quite a few to folks on here, so I have references you can check.