Every September 15, as we resist switching to long sleeves and heated seats, fuel stations in all 48 continental states must begin dispensing winter-grade gasoline under strict federal deadlines. But topping off in Chicago isn’t the same tankful as in Boise. Enter the confusingly zoned world of winter and summer gasoline.

Starting in 1989 before the 1990 Clean Air Act, the Environmental Protection Agency began restricting the volatility of retail gasoline sold in the summer. Volatility is the point at which a liquid evaporates, or vaporizes, and while that’s advantageous for a pot of pasta, it’s pretty bad for gasoline. Gasoline vapor contributes to ground-level ozone, which in turn contributes to smog and respiratory problems including congestion, coughing, and chest pains. Cars can suffer vapor lock, where the gasoline vaporizes before it reaches the fuel pump. Evaporative emission-control systems (EVAP), which use a charcoal filter in the gas tank to soak up vapors that are then mixed back into the throttle body, have largely fixed this problem since the 1970s. But when it’s hot outside, gasoline has a greater chance of evaporating from your car’s fuel system.

Federal law prohibits gasoline with a Reid vapor pressure (RVP) greater than 9.0 pounds per square inch to be sold at retail stations from June 1 through September 15. (Geeky side note: RVP is not a traditional pressure reading. Under the American Society for Testing and Materials’ D323-99a standard, one part gasoline is submerged in a dual-chambered container with four parts air in a water bath at 100 degrees Fahrenheit. A gauge measures the resulting vapor pressure in the air chamber. Therefore, the RVP measures a gasoline’s vapor pressure at 100 degrees, not at ambient temperature.)

Certain metro areas in 14 states designated by the EPA as high-ozone “attainment areas” set lower RVPs of either 7.8 or 7.0 psi. California has a 7.0-psi limit, which drops to 6.9 psi for nonoxygenated fuels. The Golden State’s perpetual summer weather means summer-grade low-RVP gasoline must be sold starting as early as April 1 and as late as October 31. Other U.S. cities such as Phoenix extend summer-grade fuels into the fall. During emergencies that cause fuel shortages, the EPA relaxes its rules. Areas of Texas swamped by Hurricane Harvey, for instance, were allowed to sell gasoline up to 11.5 psi RVP through late September.

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According to ExxonMobil, there are at least 14 unique types of summer-grade fuel sold nationwide. While the majority of states fill up with conventional 9.0-psi RVP gasoline, refiners must produce blends that are oxygenated, to reduce carbon monoxide; reformulated, in 17 states, to further reduce ozone and smog-forming toxins; for California only; and more. It gets trickier with ethanol. Since 97 percent of all gasoline sold contains ethanol—and, according to the American Petroleum Institute, ethanol blended up to 10 percent results in gasoline with a 10.0-psi RVP—the EPA counts such blends, called E10, as meeting the 9.0-psi standard. But many states, including the entire Northeastern seaboard from Delaware to Maine, don’t cut ethanol any slack. So gasoline refiners must be creative to reduce summer gas volatility, and creativity is expensive.

Whatever the blend, refiners reduce the concentration of butane—a cheaper additive with an exceptionally high RVP of 52 psi—down to about 2 percent. That means using higher concentrations of pricier additives, such as alkylates and reformates, and sometimes a lengthier refining process that slightly reduces yields. All of that—and processing those 14 special blends—helps drive up pump prices during the summer. Some states specify multiple summer blends. Others, like Arizona, use state-only “boutique” fuels year-round. This is why gas prices can vary widely across counties within just one state.

Despite those higher prices, summer gasoline contains about 1.7 percent more energy than winter gasoline. Warmup times aside, that’s why you could measure reduced fuel economy outside the summer months. Winter gasoline varies widely in volatility throughout the season. Refiners constantly adjust their RVP to as high as 15 psi—higher than sea-level atmospheric pressure, which is 14.7 psi—to help the gasoline ignite more easily in colder temperatures. The vapor pressure varies by month, region, and octane. In frigid temperatures, higher-volatility fuel is essential and does not cause the smog effect it would in the summer. Using low-RVP fuel is a nonstarter in freezing weather; the engine won’t turn over or might misfire.

It is a complicated process, but in general, gasoline refiners make the summer-to-winter switch so seamlessly that you should expect consistent, clean performance from your car’s engine regardless of the season or where you live.

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