SRW vs DRW MPG

6 wheels on the ground compared to 4 wheels on the ground is not twice as much braking surface—1.5 times as much. Extra weight from heavier dually suspension reduces mileage. In Fords a dually always comes with higher numeric rear ratio making engine spin faster causing lower mileage. GM had a white paper out a few years ago saying diesels are more susceptible to reduced mileage from high engine RPMs than are gas engines. Luckily the low RPM torque peak in diesels lets you gear them for slow RPMs if you got a big enough engine. My son went from 7.3 regular cab long bed 16 mpg to 7.3 crew cab long bed dually 12 mpg. Your results may vary.