Miles/gallon: Newer vs Older Cars

I would guess it also has to do with cost, one larger tire and wheel is still a lot cheaper than 2 smaller tires and wheels. Also just from memory of seeing big rigs on the road with dually vs. wide single tires, if you add the contact patch of the two tires together it's not much less than the one wider tire. The overall contact area needs to be above a certain amount anyway in order to support all that weight and not overload the tires.