I don't drive any of my jalopies enough to figure gas mileage. However when my father used GMCs as work trucks, we figured 15 miles per gallon. This was with 4.57 rears and 7.50x17 tires. They came with 228's but my father would buy 248 short blocks rather than overhaul the 228's and there was a noticeable change in fuel economy with 20 more cubes. We were gasoline distributors and gas was only 20 cents a gallon, but that 20 cents would buy a hamburger.
