Refineries switch over to summer-blend production in March and April. Gas stations have by June 1 to switch to selling summer-grade gas, while terminals and other facilities "upstream" from pumping stations have to switch by May 1 [source: EPA].

What is the difference between summer blend and winter blend gasoline?

Winter-blend gas has a higher RVP because the fuel must be able to evaporate at low temperatures for the engine to operate properly. Summer-blend gas has a lower RVP to prevent excessive evaporation when outside temperatures rise.

Can you use winter gas in the summer?

The main reason we can't use winter blend gasoline in the summer is that doing so would harm the environment. This is because there's more smog and ozone in the summer than in the winter, meaning there's more pollution that can damage lungs.