considering a 72 scamp

I would think it should deliver low 20's on the highway, but a lot of things can bring the mileage down on an old car. Dragging brakes, front end out of alignment, engine needing tune up and / or worn out. If the car is in excellent mechanical shape it should deliver pretty good gas mileage. You might have to spend quite a bit to get it to that point, however. Not knowing the car I can't say how this one will do in its present condition.