The Reid Vapor Pressure (RVP) is really just a measure of how easily the gasoline evaporates at any given temperature. The higher the RVP, the easier it evaporates. Simple, right? So why would gasoline producers mess around with the RVP? Why wouldn’t it be the same all year? Because: the higher the RVP rating of a particular gas blend, the easier it is to vaporize and the worse it is for the environment. All gasoline blends score an RVP below 14.7 PSI, which is normal average atmospheric pressure. Any number higher than that and gasoline would become an airline fluid, re: actual gas.
For example, winter-blend fuel has a higher RVP because the fuel must be able 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 once started, will run rough.
On the other hand, summer-blend gasoline has a lower RVP to prevent excessive evaporation when outside temperatures rise. Reducing the volatility of summer gas decreases emissions that can contribute to unhealthy ozone and smog levels. A lower RVP also helps prevent drivability problems such as vapor lock on hot days, especially in older vehicles.
In general, the lower the RVP of a gas blend, the more it costs to produce. For example, during winter, gasoline producers can blend butane — which is relatively plentiful and cheap — with gasoline. But butane cannot be used in summer as it would immediately evaporate, which is one of the reasons gasoline is more expensive throughout the summer months. On the other hand, according to the Environmental Protection Agency (EPA), conventional summer-blend gasoline contains 1.7 percent more energy than winter-blend gas, which is one reason why gas mileage is slightly better in the summer.
In addition to this, there is another reason why fuel companies are required to change over their fuel blends twice per year. If you have ever stood over the gas hose when you fuel your car, then you have probably, readily smelled gasoline. Or, similarly, if you glanced at other people using the pumps, you may have seen fuel vapors rising from another car being fueled.
As you can imagine, that raw gasoline entering our atmosphere is a powerful pollutant. Gasoline vapor contributes to the ground-level ozone, which ultimately contributes to smog and respiratory problems including congestion, coughing, and chest pains. By changing over the formulation of gasoline between winter and summer, the amount that evaporates during fueling and other handling is significantly reduced, helping to keep our air clean and our exposure minimal.
Above and beyond the national EPA winter and summer standards there are even tighter fuels regulations for cities like Houston, New York, and Los Angeles. Different states and cities have their own regulations based on their seasonal temperature. Certain metro areas in 17 states have been specifically designated by the EPA — for example, Seattle, Washington is mandated a different summer blend than Miami, Florida. In fact, there are at least 14 unique types of summer-grade fuel sold nationwide.
Processing those 14 special blends, along with higher overall demand, helps drive up pump prices during the summer. Some states even specify multiple summer blends. Others, like Arizona, use a state-specific fuel blend year-round. This is why gas prices can vary widely across states.
While it is a complicated process, gasoline refiners have been making the summer-to-winter switch for 30 years. They are well practiced and versed in the process so, regardless of your seasonal fuel type or location, you can expect a consistent, clean performance from your car’s engine.