Why would cars made from aliminium use less fuel then cars made from steel?

Why would cars made from aliminium use less fuel then cars made from steel?
Cars made from aluminium use less fuel than cars made from steel because aluminium has a low density this makes the car to be lightweight. Whereas with steel, it has a high density so it makes the car to be very heavy. So if steel was used over the same journey more fuel would have to be used and therefore less fuel would be used if you used the metal aluminium.