Well not sure what to add. But are you sure you are figuring it correct? Are your brakes dragging? What kind of numbers did you get before the rebuild? Drove mine 350 to Rod Run in Pigon Froge TN last year and got 23mpg. stock bottom end, Holley 4bbl with a header. 3.73 gears in an 8 1/4 rear, with the A/C used part time.
I bought the car and had the mechanic drive it to his house. Being that this is the most dependable engine inthe world, I didn't think it was a problem. When he got to his house, it had a couple of bent push rods.
A rebuild was needed. I have no idea what the milage was before.
I know the brakes aren't dragging. I am going to have the mechanic check timing and the carb.
I am not sure if the quality of the Mexican gas makes that big of a difference.