It’s probably a bit more complicated than this, but my father in law tried to tell me there was no difference between summer and winter gas. I looked it up and from what I found Butane is a common additive to gasoline and little to none can be used in the summer time because of its vaporization point. They can put more in the gas in the winter time due to lower temps…and that’s why prices drop a bit in the winter too. The gasoline is less pure and so gas mileage drops.