I know black smoke is unburned fuel. When I keep my foot light I get close to 23 MPG. Not bad for a 3/4 ton truck not towing. I also get about 700 miles from a 36 gallon tank. I only blow black when flooring to pass. I have a friend that has a Ford with a PSD. His won't smoke no matter how he drives. But he also only gets about 16 MPG. When I floor it it causes too much fuel to be sent to the cylinders. So some is going to come out unburned as smoke. Take a look at most 18 wheelers as they are gearing up. I have a coworker that has twin upright exhausts on his 1 ton duallie Ram. You should see the black smoke he puts out accelerating.
"I won't be wronged I won't be insulted I won't be layed a hand on. I don't do these things to other people. And I require the same from them." (John Bernard Books) John Wayne The Shootist his last Movie.