hmmm id say go with a 4 banger if you want mileage lol....but on your motor id go with low compression a small cam,a small carb in the 300 to 500 cfm range and either Denso or NGK iridium spark plugs...mileage also depends on what type of gearing your vehicle has too,,,low geared is good in town and for pulling and high geared is good on the highway so id pick something in between 355 is a good all around gear or 373 is too