noob needs help with mileage - only 10 MPG!

I was originally getting 10mpg back when I had a remanufactured Holley 1920 on there. Swapped it with another remanufactured 1920 and it jumped to 12mpg. At that point I said screw it and went with a Super Six with a NOS Carter BBD and it jumped to 18mpg. I threw on an HEI ignition and a 2 1/2" exhaust and I've been 21-22mpg ever since.