I built a 400 pump gas stroker motor(511 C.I which ended up becoming a 518 C.I. later ) with a six pak for my 1971 Duster, it was low compression and ran on CA pump swill with no problems when I dyno tested it. Bigger C.I. is better, more torque, mine made 644 Ft lbs at 4500 RPM originally at 511 C.I. More torque equals running the motor slower so you end up with better gas mileage, if you can control your right foot I never tried to figure out the gas mileage as it was not my dailey driver, I had other vechicles that got excellent fuel mileage around town That rascal sure was fun to drive, it went 9.993 at 134.63 MPH with the full exhaust on to the rear bumpers and the air cleaner on with me in it weighing 3450 Lbs. I'm building another street car now, it is a 1966 Dodge Coronet Deluxe two door sedan with a pump gas 464 C.I. 440 motor with a NP833 O.D. tranny with a Dana 60 with 3:55 rear gears for cross country driving, maybe next year I will go terrorize some of the local car show guys who think they have fast cars becuase they paid a lot of money for someone else to build there cars for them