diesel engines are like a gas powered motor, but a lot bigger and produce more power. But the biggest difference is the fuel that it runs to operate. that is diesel fuel, this is a less purified version of gasoline. that is why you see these motors rolling a black cloud behind them because it has more impurities in it than normal gasoline does. but what fuel is more efficient?
The reason why they use a diesel engine in big trucks like this is because of the power that a diesel engine can produce, and the efficiency. They can haul a bigger load because of the power that the diesel provides.
This graph shows the consumption of gas and diesel engines. As you can see diesel are more efficient on fuel then the gasoline. But through the near coming years you can see gas is predicted to have almost the same efficiency.