I will tell you this: Extended / King cabs are bad for fuel economy. You want fuel economy then don't get one with a back seat. Engines much over 5 liters (~300 cid's) are not so good either, you want decent fuel mileage then that engine comes in under 318 cid's or under 5.2 liters, no two ways about it. Air conditioned luxury trucks (SLT's / XLT's) bad. Get a standard truck, no frills, no thrills. Short or standard beds are far better than the long bed as well. Every foot of truck adds 500-800 pounds of weight. Height is bad, the higher the truck sits, the worse your fuel economy will be. Avoid buying raised trucks, avoid raising them, stick with OEM shocks and springs and wheels and tires. Inflate your tires to Max.PSI sidewall, check them often. Start learning how to drive with load. (Certain hypermiling techniques such as Driving Without Brakes might come in helpful as well).