Diesel

Diesel engines ignite fuel with compression heat rather than spark plugs. They extract more energy per gallon than gasoline engines, which historically meant 20 to 30 percent better fuel economy, especially on the highway.

Their abundant low-end torque makes diesels natural tow vehicles, which is why they persist in heavy-duty trucks. In passenger cars they have largely given way to hybrids, but used diesel SUVs and trucks remain sought after for towing.