I've never managed 18 mpg. Usually for me, in a 3/4 ton truck with a 235 ci straight six, I was getting about 16 mpg. But then again, I was running with a 4.56 rear axle, so maybe folks with half tons DO get better mileage than that.
You would probably get better mileage with a fuel injected V8 than you would with a much weaker straight six.
