When a 460 carburated motor gets better gas mileage than a 390 is when we will see 25 cent a gallon gas again, it won't get worse either. Will it out preform the 390, yes. BTW all 460s came with a 4 barrel carb and they are all junk and need to be replaced. Oh yeah, at this time I have a '68 F250 Camper Special with a 390 4 barrel and C6 and an '84 F250 Supercab, Superduty with 460 4 barrel and C6, so I have compared.