Does 1970 chevrolet trucks get good gas mileage?

No, 1970 Chevrolet trucks did not get good gas mileage by today's standards. They were large, heavy vehicles with relatively inefficient engines compared to modern trucks. Expect very poor fuel economy, likely in the single digits MPG (miles per gallon), even with careful driving. The exact mileage would depend heavily on engine size, transmission type, vehicle condition, and driving style.