If you are going to buy a diesel just for mileage, then you probably shouldn't get it. (I know this isn't the case though) There is a wide variance in mileages that people get with their trucks. My friend has a 02 F350 with the 7.3 engine in it and he gets just the same mileage that I do with my 04 GMC sierra 2500HD with LB7 engine. We both have 285/75-16s on. Driving unloaded on the highway (or what they call it here in AK) I get about 18-19 MPG. When I tow my friend's 24' enclosed trailer with 4 snowmachines, I have gotten 10-14MPG. The only difference in mileage came from the difference in temps outside. If you were to get a GM truck, I would go with an 03 or 04 with the LB7 engine. The LLY engines are hit and miss with overheating problems. The only problem that the LB7 engine has had is injectors. They are covered to 200,000 miles on a case by case basis. As far as the Fords, as long as you get one that doesn't have the 6.0, you shouldn't have to worry about tranny problems.