Im not really worrying to much about gas mileage anymore.I have come to my senses and I do not want to spend Thousands of dollars on a motor in which the car I drive is not my every day car.The one thing I can use in the future is a great set of aluminum oval port heads that will make at least 50 more HP.I have no idea what my gas mileage would be with my 402 Big Block because my gas gauge doesn't work.One thing is that distinct sound of the Chevy Rat Motor Big Block can't be duplicated with a LS1 or 2 motor.With 4:11 gears 28 inch tall tire,big 595 solid lift cam, I would say between 8 to 12 miles per gallon???12-19 miles per gallon for a big block is hard to believe or do you have a small block?? What size engine do you have and what are your rear end gears??