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Fox News Weather Center

Winter Vs. Summer: When Is Gas More Costly?

Gas prices sway throughout the year -- even on a daily basis -- but not many people realize that weather contributes to the cyclical nature of gas prices.

Some commuters may notice that they generally pay more per gallon of gas during the summer, and this is partly because summer gas is more expensive to make than winter gas.

There are different blends of gas that are sold based on the time of year.

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Prices tend to begin a steady increase during a maintenance period toward the beginning of the new year, when refineries prep the summer-blended gas, which is required to hit the markets by May 1. Prices are also generally more expensive throughout the summer, as refineries continue to make this more expensive gas blend.

Summer-blended gas contains more costly compounds that burn cleaner, working to reduce smog during the hot summer months, said Gregg Laskoski, a senior petroleum analyst from GasBuddy.

The compounds and hydrocarbons that make-up a gas blend dictate the temperature at which the fuel will burn and evaporate, allowing the car to function properly. For example, at the same temperature, a gas with a high vapor pressure will vaporize more readily than a gasoline with a lower vapor pressure. Therefore, gas blends have to accommodate the outside temperature for the car to function properly.

According to the EPA, the volatility of gas, which determines how likely the fuel is to evaporate, has to be adjusted for the climate and time of year. In northern areas during the winter, gas is required to contain more high vapor pressure compounds, which will allow it to evaporate at a lower temperature. In the hot summer, gas is required to have fewer high vapor compounds.

If winter-blended gas was used in the summer, it would overwhelm a vehicle's emission control system, which contributes to summertime ozone, according to the EPA.

Volatility or vapor pressure is measured using the Reid Vapor Pressure (RVP) system. Summer-blended gas has nine RVPs or less and winter gas has between 11 and 15 RVPs, Laskoski said. Winter-blended gasoline contains more butane, which has a very high RVP, but is also a less expensive additive.

Even though winter-blended gasoline is cheaper, consumers may notice that they head to the pump more often in the winter than summer after the same amount of driving. According to the U.S. Department of Energy, winter-blended gasoline can have slightly less energy per gallon than summer-blended gas.

However, other winter-related factors can impact a car's gas mileage. For example, engines take longer to heat up and reach their fuel-efficient temperatures after being enveloped in winter's frigid air.

"A conventional gasoline car's gas mileage is about 12 percent lower at 20 F than it would be at 77 F," according to the U.S. Department of Energy.

Generally, people also believe that letting a car warm up before driving is important for a car's longevity. Consumers are also more inclined to idle and make use of the car's amenities including heated seats, heater fans, four-wheel drive and window defrosters during the winter, which all use additional power.

Plus, cold air is denser, which increases the drag on a vehicle. Tire pressure also decreases in colder temperatures, which increases resistance. And, snow-coated and icy roads also decrease the tires' grip, which wastes energy, according to the Department of Energy.

How to Improve Your Winter Gas Mileage:

1. Check your tire pressure regularly.

2. Park your car in a warm place to increase the initial temperature of your engine.

3. Combine trips so that you drive less often with a cold engine.

4. Minimize idling your car to warm up. Most manufacturers recommend driving after 30 seconds.

5. Don't use heat warmers more than necessary.