vurtok hit it dead, on... generally 4 barrel carbs will give you better gas mileage than 2bbl carbs. as long as your secondaries are vaccuum and you keep your foot out of it of course... mechanical secondaries destroy mileage... most i can tell you is that if you want to keep it so if you do open up your secondaries you dont eat up too much gas, would be to set it up so when you do open them up, that the butterflys dont open too far, which is adjustable on most carbs...