I have a big heavy F-350 with the 7.3. It gets 20-23.8 mpg empty.
My uncle Alaska Bob was a diesel mechanic for decades. He taught me the importance of getting air in and out of a diesel engine. After market air intakes (not just a different filter) and a new larger 4 inch exhaust system make a big difference.
The head pipe on my stock exhaust was 3 inch in diameter except there was a 90 degree bend that flattened the pipe. Replacing it with an after market 4 inch pipe that is totally round changed the performance of the truck.
