CHARLESTON, SC (WCSC) - You may not notice a difference, but the gas you are now putting in your car is a different blend from what you were using just a few months ago.
Not only does gas come in three different grades (regular, plus, and premium), but it also comes in a winter and slightly more expensive summer blend.
According to the U.S. Energy Information Administration, refineries switch the mixture twice a year, once in the fall and once in the spring. The difference between the two blends is its volatility, known as its Reid Vapor Pressure (RVP).
Lori Jo Simmons with Summerville AAA says the RVP is measured by pounds per square inch, and the higher the number the more likely it is to evaporate.
Summer gasoline has a low RVP, making it less likely to evaporate when compared to winter blends high RVP.
"In the summertime, all refineries are just focusing on a cleaner burning fuel for the United States," said Simmons.
Refineries must transition to the summer blend by May 1, while gas stations have until June 1 to start selling it.
Refineries switch back to the winter blend around the beginning of September.