I had a 67 Dart GT vert and a Spirit of 76 Dart that I drove for several years back in the 80's. They both got 16mpg. I had a 318 Charger and a 318 Coronet, the Charger got 18mpg and the Coronet 21mpg. My son,s 66 Dart SW when it had a 318 got 22mpg. These are all highway miles. In perspective if I drive my GTX nice, it gets 16.5 with the 440-6. Although normally I'm at 14 on it. My V6 Dakota gets 20mpg and my polyspere 318 65 9 pass wagon gets 19.