Old gasoline causes poor performance and can even damage a vehicle's engine. Gas loses volatility over time and can also become tainted when moisture condensation increases its water content.
During refining, oil companies mix gasoline such that it contains the proper volatility, or a measure of at what temperature and pressure the gasoline ignites. When gasoline is stored, it gradually loses volatility. When burned, the gas does not ignite fully if an engine is otherwise operating normally.
Deposits from unburnt fuel can cause engine damage. Gas also pulls water from ambient air that can damage a car's carburetor or fuel injection systems. AOL Autos recommends letting fuel sit in a car's tank for no more than two months.