You gain mileage red light to red light with the taller gear but lose on the highway. Its a trade off for what best suites you thats why its a option. To sorta answer your question 4.88 in a 04 6.0 f550 around 11mpg pulling 10k lbs and 4k in the bed plus the steel body. 50/50 mix of highway and city plus a lot of idling about a hours worth on a tank.