Sort of an extreme example but I'll throw it out there-

I went from a 119,000 1974 318 with a 4-bbl and duals to a '94 roller cammed 5.9 with H.S. roller rockers and the MPG got better.

Much less throttle is needed to get the car up and moving and the later model motor has much less friction with the roller cam and rockers plus lighter pistons and rods with a windage tray too.

Same carb, exhaust manifolds, distributor.

Of course at WOT, that is another story..

Been several weeks since I did the swap, very happy as I expected to get punished bad by the MPG gods but it didn't happen..