Ranchero , I have a 69 mach 1 with a factory 390 in it and when I checked my fuel mileage I got 18 to 20 mpg (that's when the pedal isn't to the floor) and I'm sure it has 3:55 gears in it to. As far as hp , I'd say at least 350 hp although alot of guys say that's wrong but you couldn't prove that to me. Don't worry it'll fly down the highway if you want it to.

Rocky