In my stock 318 '72 dodge dart with a 4bbl eldelbrock intake, 600cfm holley, and headers/dual 2 1/2" exhaust with 2.76 gears I get 15-16mpg going 80mph. I get worse mileage at 60mph compared to 80mph. at 60 im barely turning 2,200rpms. At 80 im turning 2,900-3,000rpms.