We'd get 20 MPG with the '29 Tudor running Sunoco regular, in the 70's. Driving 40 MPH average.
My Dad had a 1960 Dodge that he bought new, (why he bought that thing I'll never know, got rid of it soon after and went back to Fords) and struggled to get 12 MPG. Guys at the restaurant he frequented that had '59 and '60 cars of that same ilk bragged about getting 18 and 20 MPG. Pop took that thing into the local Mopar garage several times complaining about the gas mileage, they timed and fiddled and fooled with it, and it never got better. When he told the Service Mgr. about the gas mileage the others were getting, the Serv. Mgr. told him 'The difference is, YOU'RE telling the truth.'
I never forgot that lesson he told me

And I was just a kid back then.