I just bought my first diesel truck back in November. It's a 2005 F-250 with the 6.0L and the 5 spd torq-shift automatic tranny. Last weekend I drove the truck to South Carolina, about a 600 mile round trip from where I live. On the trip the truck only got 13 mpg! I wasn't towing a trailer and the truck was empty except for me and my suitcase. It's not loaded up with mods or anything, it's completly stock power wise. I'm extremely dissapointed, esspecially since a friend of mine has a 2006 F-250 identical to mine and he gets up to 21 mpg on his. Can someone tell me what can be done to improve my mileage? I'm not a lead-footed driver, and the trip was done with almost all interstate and highway driving, very little driving on backroads or stop and go. If this is the best the truck will get, I'm going to sell it and buy a F-150!