With all the emissions regulations these diesels will never get the mpg's they used to. However, some tuners seem to be getting great mileage out of their 6.4's with a lot of
emission "deregulation". Matt at Spartan is claiming 21+ mpg's at 80 mph. I wish my truck would do that. Just means you have to sacrifice warranty for mpg's, but a 7 mpg increase will pay for itself after awhile.
Good news is all the new motors are common rail which is much better than the old HEUI.