I had a 59 gmc 1/2 ton with a 270, 3 on the tree and 3.07 rear gear. I got about 16 mpg till they reformulated the California gas back in the 90s. Then it dropped to about 14 mpg. It was about that time I put a 4bbl intake onand a holley 390 cfm carb. Gas mileage stayed the same but that 270 was really starting to wake up. I didn't have any other mods to that engine like cam or split exhaust or HEI... just a stock Jimmy motor with an aftermarket 4BBl intake. That truck was mostly driven on the highway at about 70mph. If taking the backroads I imagine mileage may have been in the 19 mpg range, but let's face it... a 1959 gmc is not the most aerodynamic vehicle on the road.
My current project is a 1940 gmc with the original 228. That engine will be coming out soon to make room for a heated up 302. I don't think the 1940 driveline will hold up to well to 225 HP, so I am swapping out everything behind the engine.
Gearing and driving speed have a lot to do with fuel economy. Carb choice can affect economy as well. If you want to build a rocket well you can't get off the ground without burning some fuel.
