Gas stations start selling summer-grade fuel, around Memorial Day weekend. This marks the beginning of the summer travel season.

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Why is summer fuel more expensive than winter fuel?

Unfortunately for drivers, gas prices often go up during the summer, starting around Memorial Day [Source: EPA]. There are many reasons behind the increase in summer fuel prices, and some are fairly logical. More people traveling, especially on family vacations and road trips, increases demand. Also, in the spring months, energy companies conduct maintenance on their refineries, shutting them down and limiting capacity until late May. Because of these disruptions, oil supplies can become stretched. In addition, natural disasters, like hurricanes, can increase prices by disrupting transport routes and damaging refineries and other infrastructure.

But did you know that the gasoline sold during the summer is actually different -- and more expensive to produce -- than that sold in the winter? In this article, we'll take a look at why summer fuel prices are higher, focusing on the annual shift from winter-grade fuel to summer-grade fuel.

Twice every year in the United States, the fuel supply changes. It's known as the seasonal gasoline transition. This change is the biggest reason for the price hike in summer gasoline. Depending on the time of year, gas stations switch between providing summer-grade fuel and winter-grade fuel. The switch started in 1995 as part of the Reformulated Gasoline Program (RFG), which was established through the 1990 Clean Air Act Amendments. The Environmental Protection Agency (EPA) started the RFG program in order to reduce pollution and smog during the summer ozone season, which occurs from June 1 to Sept. 15 [Source: EPA].

In order to reduce pollution, summer-blend fuels use different oxygenates, or fuel additives. These blends, the EPA claims, burn cleaner and also help compensate for a limited oil supply. The EPA says this practice of using seasonal blends also encourages the development of alternative fuels [Source: EPA]. (Remember that gasoline isn't just made up of processed crude oil -- it's a blend of refined crude oil and different compounds and additives.)

So what's the difference between summer-grade fuel and winter-grade fuel? Summer-grade fuel is more expensive for two reasons -- because of the ingredients it contains and because refineries have to briefly shut down before they begin processing it. Summer-grade fuel also burns cleaner than winter-grade fuel. This just means that it produces less smog and releases less toxic air pollutants, which we'll talk about more [source: EPA]. The actual difference in cost of production varies. One estimate claims an increase of only 1 cent to 2 cents per gallon [Source: Slate], while another states 3 cents to 15 cents per gallon [Source: Reason]. No matter the difference in production costs, the increase at the pump is even greater, owing to the summer driving season, dips in supply, maintenance costs and companies' converting to production of summer blends.

On the next page, we'll take a look at why summer-grade fuels are more environmentally friendly and when exactly the shift between summer and winter fuels occurs.

Oil refineries like this one in California shut down for a few months every year. This is another reason why summer fuel prices are higher than winter fuel prices.

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Summer-grade versus Winter-grade Fuel

During the summer, pollution is a frequent concern due to increased levels of smog and ozone, which can harm the lungs. Summer heat boosts the formation of ozone, while the appearance of an inversion layer -- an immobile layer of air -- can trap pollutants in the lower atmosphere [source: EPA].

Summer-grade fuel has a different Reid Vapor Pressure (RVP) than winter-grade fuel, which contributes to its being (marginally) more eco-friendly. RVP is the vapor pressure of gasoline measured at 100 degrees Fahrenheit. Fuels with higher RVP evaporate more easily than those with lower RVP. A particular fuel blend's RVP is based on the combined RVP of the ingredients that make up the blend. Regulators worry about this evaporation because it contributes to ozone formation.

Gasoline must have an RVP below 14.7 PSI (pounds per square inch), which is normal atmospheric pressure; if a fuel's RVP were greater than 14.7 PSI, excess pressure would build up in the gas tank, and the fuel could boil and evaporate. Depending on the part of the country, the EPA's standards mandate an RVP below 9.0 PSI or 7.8 PSI for summer-grade fuel. Some local regulations call for stricter standards. Because of these varying RVP standards, up to 20 different types of boutique fuel blends are sold throughout the U.S. during the summer [Source: Slate].

Because RVP standards are higher during the winter, winter-grade fuel uses more butane, with its high RVP of 52 PSI, as an additive. Butane is inexpensive and plentiful, contributing to lower prices. Summer-grade fuel might still use butane, but in lower quantities -- around 2 percent of a blend [Source: The Oil Drum].

We know that gas prices go up during the summer, generally around Memorial Day, but when do companies start producing these different summer fuels? The EPA defines April to June as the "transition season" for fuel production [Source: EPA]. Refineries switch over to summer-blend production in March and April [Source: EPA]. Gas stations have by June 1 to switch to selling summer-grade gas, while terminals and other facilities "upstream" from pumping stations have to switch by May 1 [Source: EPA]. Following the summer driving season, companies switch back to winter blends beginning in September, with the first winter increase in RVP allowance occurring on Sep. 15.

In a 2001 report, the EPA claimed that "roughly 75 million Americans breathe cleaner air today due to [the seasonal fuel] program" [Source: EPA]. Still, the increased price, combined with the use of controversial additives like ethanol (which is less energy efficient than gasoline and produces more smog) and methyl tertiary butyl ether (MTBE), means that the program may still have its detractors.

To learn more about summer fuel prices and related topics, explore the links on the next page.

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Lots More Information

Related ArticlesMore Great LinksSources
  • "Guide to Federal and State Summer RVP Standards for Conventional Gas Only." Environmental Protection Agency. April 15, 2008. http://earth1.epa.gov/otaq/volatility.htm
  • "Study of Boutique Fuels and Issues Relating to Transition from Winter to Summer Gasoline." Environmental Protection Agency. October 2001. http://www.epa.gov/otaq/regs/fuels/r01051.pdf
  • "The Plain English Guide to the Clean Air Act: Cars, Trucks, Buses and "Nonroad" Equipment." June 11, 2007. http://www.epa.gov/air/caa/peg/carstrucks.html
  • Bailey, Ronald. "Gasoline Prices: Conspiracy or Plot?" Reason Magazine. March 23, 2007. http://www.reason.com/news/show/119300.html
  • Patterson, Dan. "Why Do Gas Prices Go Up In the Summer?" ArcaMax. April 3, 2007. http://www.arcamax.com/automotive/s-178668-228995
  • Rapier, Robert. "Refining 101: Summer Gasoline." The Oil Drum. March 16, 2007. http://www.theoildrum.com/node/2374
  • Rapier, Robert. "Refining 101: Winter Gasoline." The Oil Drum. Sept. 15, 2006. http://www.theoildrum.com/story/2006/9/13/234043/431
  • Schechner, Sam. "What Is Summer-Blend Gas?" Slate. April 12, 2004. http://www.slate.com/id/2098672/