The ones I drove got between 12-18 mpg. '69 Catalina/'69 Firebird, etc. Had a '61 Chevy belair with a 235 cu 6 in it and it was lucky to get 20 on the highway. You must have run into some extraordinary full size '60s cars.
In the 60s, the only benefit to getting a smaller engine was the car was cheaper to buy. Smaller engines didn’t cost much less to operate and they wore out much quicker. Most people wanted the biggest engine they could get.
This is the type of car I was talking about, and not with the powerglide slushbucket...I've heard and seen 21 mpg or better.
If you consider the weight of the Bel air, which had a full frame, and compare it with modern cars, I think it fares pretty well. The V8s and automatics not so much, and certainly not the muscle cars.
The advent of fuel injection could have made cars very fuel efficient were it not for overkill by the emissions freaks.
EFI, electronic ignition and the converter all vastly improved the driveability, fuel mileage and emissions of any gas powered vehicle. Other "improvements" did not.
The "smog pumps" AIR tubes, miles of vacuum line and closed loop carburetors were all crap and have for the most part disappeared from production vehicles.
I fully believe that a good portion of the fuel consumed in today's cars is not for the operation of the engine, but to optimize the burn in the converter, hence the downstream O2 sensors.
If they were to simply shoot for the leanest possible fuel ratio at any given time, without compromising performance when needed, nor jeopardizing combustion chamber components, they would have accomplished the same or similar emissions results.
After all, it would seem that the less fuel consumed per mile, fewer emissions would be created.
Aerodynamic are a big plus (but often result in some butt-ugly cars) and the ubiquitous automatic is a minus on mileage.
My point in a nutshell; currently the producers are forced to tune for emissions, your mileage and wallet be damned.