You mean a 70s truck where a 1 ton version won't even tow what a 1/2T can now?
I had a mid 70s 3/4T truck with a 454. 240HP and 370 ft lbs at 3800 RPM. It got around 8 MPG
My 2018 F150 has the 3.5 ecoboost engine. It has 375HP and 470 ft lbs at 2500RPM and gets 20MPG on the highway.