Interresting discussion here on a new box... now we all know how to get more HP and torque. . more air, more fuel, timing, boxes, Nos. . etc...
there are 3 factors for mileage . . weight, speed, aerodynamics. . actually 4. . and weight to power ratio.
I drive a big rig and know that if you have more power for the load, it will get better mileage. .
a test was done on a dodge. . it was a 2wd, did some side skirting for the tires, and other stuff, reduced weight, aired up tires to max psi, and got close to 30mpg.
I have a 01 2500 4wd, my 59 was rebuilt to . 020 over, I have a hybrid turbo, 6 speed manual. and in the warm months I run 245 70 R 19. 5" truck tires. mainly for towing but they roll great solo. I am constanly seeing over 20 mpg when cruising under 60 mph and sometimes over 30. . engine is turning 1500 rpm at that time. . mind you this is flat road no hard push. .
wind resistance takes over the rolling resistance as determining factor over 55 mph. so go slower and coast when you can and you will see better mileage.