Gas prices change throughout the year, especially as seasons move from fall to winter and winter to spring. But is there really a difference between the gas available in the summer versus the winter?

The difference between the two blends involves the Reid Vapor Pressure (RVP) of the fuel. RVP measures how easily the fuel evaporates at a given temperature. The hotter gasoline becomes, the faster it evaporates.

Winter-blend fuel has a higher RVP because the fuel needs to evaporate at low temperatures for the engine to operate properly, especially when the engine is cold. If the RVP is too low on a frigid day, the vehicle will be hard to start and won’t run as well.

Summer-blend gasoline’s lower RVP prevents too much evaporation when outside temperatures rise. Reducing the temperature of summer gas decreases emissions that can add to unhealthy ozone and smog levels.

According to the Environmental Protection Agency (EPA), typical summer-blend gasoline contains 1.7 percent more energy than winter-blend gas. That’s one reason gas mileage is slightly better in the summer. However, the summer blend is also more expensive to produce. That’s why gas tend to be more expensive during warmer months.

AAA tracks and reports gas prices across the country. See the most and least expensive markets and plan for road trip expenses by visiting the official gas prices website.