Gasoline

Gasoline is made up of a lot of different chemicals, with many different boiling points, vapor pressures, and ignition temperatures. High vapor pressures are good for evaporating the gasoline droplets quickly and mixing it thoroughly with the air, but high vapor pressures also generally mean lower combustion temperatures which cause knocking, and also get government pollution dweebs all excited about air pollution caused by gasoline evaporating out of your gas tank.