Yes, they can! Why? It is the physics related. Because of massive weight truck have to haul, once going on cruising on the highway, there is little need for power. Don't you realize that it takes longer for 18 wheelers to brake to full stop. It is all because of the weight in the back. Same goes with the train, it takes 1 mile for train to go down from 55 MPH to full stop. That is why train was able to use the least energy to haul 400 cars with 2 to 4 engines. On other hand, cars are very light, and they have more horses than really needed, and because gasoline engine don't vary with horses like Diesel engine do. What I mean Diesel engine can drop from 70hp to measly 30hp once on cruise (That is for 4 cylinder). That is HOW they can go higher MPG.
Now, you see why 18 wheelers takes only 8mpg in the city, it is because truck have to stop and go and stop and go, this uses too much energy to pull heavy load.
My dad was truck driver, and that I once own diesel powered car. That is how I know about these stuff. The only big problem with diesel is that their soot is cancer causing agent. The researcher just discovered about a year ago, which really disappoints me. Diesel engine has plenty of advantages. They are actually oil, thus kepts the engine lubricate all the time. That is how Diesel engine lasts much longer than gasoline engine (Gasoline is vapor not oil!), there is more to advantage of this diesel, need not to list more. grin.