F550 weighs more than a F450. Depends also on cab config, bed options and length, etc. 4-5mpg seems low. You can get a class 8 that gets better mileage. Something doesn't sound right. My old F450 V10 with 11' service body and 4.30 gearing got 9mpg on a bad day. Thus, the smaller diesel should easily get 10+. Running a low geared truck on the highway is the wrong way to do it. You have to compromise otherwise an aftermarket two speed axle would be the way to go for maximum economy. If your engine runs over 2500rpm at 65-70mph highway, you're over-geared and just wasting fuel. Loaded, you should be in the 2000-2300rpm range so there's enough flexibility for you to downshift to maintain momentum (speed) on the highway.