I agree that the new engines are down on mpg but another big factor is weight. I have a 1999 F-250 and a 2004 F-250 that are both diesels. One is a 7.3 and the other a 6.0 The trucks are both short bed, extended cabs, 4x4's. The later truck weighs nearly 800# more.
This is true across the board. I had a 1986 Ford Escort that got 30+ mpg. I bet that car did not weigh 2,000#. I just picked up a 2005 Saturn and it does 32 mpg. 20 years difference, fuel injection, better tranny did not help with the mpg. Why? The Saturn has things like crumple zones, air bags, ect. All this adds weight and this car tips the scales at 3,000#
Chris