Most of the medium duties will average the same milage.
So there are no exact numbers for a Freightliner vs a Ford.
We have a F-650 with a Cummins, 6sp manual. It gets about 7mpg. Sometimes more.
And really its not so much the engine...its the gear ratio that makes the difference in milage of gasoline.
So many different configurations.
As well as the body. Does it have a full cab protector? And so on.
Really what a buyer needs to be concerned with is things like gear ratio, and CAB to AXLE measurements. Improper cab to axle measurements can cause your truck to eat steering tires if the weight is not properly and evenly dispursed on both the front and rear axle.
Also, dealers are quick to try to sell contractors on Allison transmissions. I had one sales rep tell me "this is the same trans they use in the ambulances!" Well, BFD! When was the last time you say an ambulance loaded with soil and pulling a 7000# skid loader at the same time?!! So you need to make sure the trans will also hold up to a fully loaded truck that is pulling a 3000# trailer with a 7000# skid steer on it.
These trucks are expensive. Annual DOT inspections. And big repair bills.
If you're serious bout a medium duty (26,000 gvw and up), then fuel economy needs to be the least of your concerns!
MD is huge on enforcing DOT laws. Are you up for dealing with that?
Since you're in Annapolis, Cummins Power Systems is in Glen Burnie, which means you're not far from having a full fledged manufacturer authorized facility to service it / work on it.