Juke owners

Rockhound

Nope, he has it right. There are distinct seasonal blends and their sale is strictly regulated by the EPA. TL; DR...winter gas is of lower energy content and can contribute to lower MPG.

The blends differ in their Reid Vapor Pressure (RVP). RVP is the vapor pressure measured at 100 deg F; a higher RVP means that blend evaporates easier.

Colder, denser air in winter allows for fuel with a higher RVP, so additives such as butane are blended into winter gasoline. This means winter-blend gasoline yields less energy per unit volume than summer gas.

The EPA suggests this energy decrease is negligible, but it appears from empirical data that some cars are more sensitive than others, just as with ethanol and decreased MPG.

The other factors mentioned above obviously all weigh in, too, all conspiring to make fuel efficiency decrease in the winter months.