More fuel without extra air means black smoke. It's all about optimums, intake air filter cleanliness, air intake temperature, height above or below sea level, fuel delivery pressure, both low to the pump and high to the injectors, injection time and duration, and compression peak pressure. Then the person on the controls makes it talk or not. I only used to get 10 out of a Scania 420 auto running on 6 axles pulling a curtain sider over the hump to Manchester from the North East, It was usually fully freighted both ways. I could get 17 out of a manual ranger plus splitter 460 CF Daf running nearly the same route pulling a reefer but only on 5 axles running 38 tonnes down and 22 back. With a 420 Scania auto it was 12. With a C6SFL in a Vickers Vigor I used to use between 5 and 7 gallons per hour on dozer work, it was about the same pulling a 9/11 box. Depending on ground conditions.