For increased mileage purposes a 600 CFM carb will improve things. Besides that if you still have the stock cam, anything past 4500 rpm is just beating your motor up for no good purpose. My 1984 F250 Supercab came with the junk 4180 600 CFM Holley emissions carb. I replaced it with a 4150 600 CFM List #1850 Holley and gained low end performance and almost 2 mpg. Along with a dist. recurve and a 3" single exhaust it now gets almost 10 mpg and that is with 4.10 gears in the rearend.