My 2001 F350 SRW Crew Cab Lariat V10, which I sold last month, because I didn't need it any more, got 10-11mpg carrying a 3,000 lb TC. Best mileage I ever got was 15.6 on the interstate, unloaded, doing 65mph. I averaged about 12 mpg overall. I think there may be something wrong with your truck. I concur with those above who suggest checking filters, brakes, tire pressure, exhaust system, etc. My truck was stock, and I bought it when it had 114K miles on it. The only problem I ever had was that it spit a spark plug pulling a grade at 3,800 RPM (a common problem with the early V10s). If I needed that much truck again, I would buy another one in a heartbeat.