Fuel composed of petroleum distillates which has a boiling point and specific gravity higher than gasoline. Diesel fuel is generally the primary fuel source used in medium and large transit vehicles, while smaller vehicles often as gasoline powered. Diesel vehicles typically have better fuel economy than equivalent gasoline engines and produce less greenhouse gases. However, diesel fuel often contains higher quantities of sulfur and is now subject to more stringent emission standards with the transition to ultra-low sulfur diesel (ULSD).