What kind of gas mileage would a 1980 El Camino V8 get?

A 1980 El Camino with a V8 would get very poor gas mileage by today's standards. You'd likely be looking at somewhere in the range of 10-15 miles per gallon (mpg) in combined city and highway driving. Highway driving might stretch it to the high teens under ideal conditions, but city driving would probably be closer to the lower end of that range or even worse. The actual mileage would depend heavily on factors like engine size (the larger the engine, the lower the mpg), driving habits, vehicle condition, and tire pressure.