I don't know about the 6.7 but when I had my 7.3 it would get about 18 mpg empty at 65 mph or less. At 70 mph I would lose 2-3 mpg. Once the ULS fuel came out with the higher price I didn't see any savings over gas. Granted, if you tow a lot a diesel might be better.
2001 F250 CC PSD (sold!)
2014 F250 CC 6.2 (current!)