Friend's /6 '67 Barracuda ran 20 in the quarter. Put a 2bbl and header on and ran 18sec in the quarter, but his gas mileage dropped from somewhere in the 20s, to 14mpg. Went 360, gas mileage stayed the same, but now in the 14s. Then went 440, and same mileage, in the 14s still, but a lot faster MPH. He couldn't hook, but a lot more fun to drive. These were all with bone stock, worn out motors, except for the /6 header.