Tell your friends to invest in a good chip and couple upgrades! My old (by today's standards) 97 Powerstroke has some rather cheap upgrades, Superchips Tuner, Turbo and Intercooler from 99+ powerstroke, intake, and less restrictive down pipe from the turbo. I can honestly say that I have done close to double the mileage your buddies are gettin with a hell of a load. I don't drive like a mainiac with all that weight either. Let them know, that is if you want to lose your bragging rights.
All in good fun,