Have you ever noticed that U.S. gas prices are cheaper on average in wintertime than summertime? Have you got a pet theory as to why that might be? It turns out it’s not a case of market forces trying to entice hibernating drivers out into the cold to hit the road and burn fossil fuels. Instead, it’s due to the different formulations of summer and winter fuels, with the formula for winter fuel being cheaper, sometimes by as much as 15 cents per gallon at the pump.

Winter gas prices 2

The different grades of fuel have been in effect since 1995. In fact, there are around 20 different formulations of gas in use in the U.S. depending upon location. The use of summer and winter blends is enforced under the EPA’s Reformulated Gasoline Program (known as the RFG). Gas stations must sell summer fuel between 1 June and 15 September (and for an even longer period in some hotter states), but can begin the springtime transition from April onwards. The transition from one blend to another is expensive for refineries since they have to shut down production temporarily, and this explains trans-seasonal spikes in prices, particularly around April.

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The different gas blends are designed to reduce air pollution and emissions caused by the prevailing temperatures. In summer, regular gasoline evaporates more easily due to the heat and mixes with airborne particles to create ozone, which, of course, no one wants any more of than can be helped, thank you! Chemical additives are used that reduce the vapor pressure of the summer blend, making it less likely to evaporate. The mix also uses less butane than the winter blend. However, while light, butane is relatively cheap, and when the vapor pressure requirements are relaxed in winter, more butane goes into the gas blend, allowing the price to drop. In some areas that experience high pollution levels, winter-blend fuel must also contain oxygenates — usually alcohol or ether. These reduce vehicle carbon monoxide emissions that can otherwise increase in wintertime.

Sadly, due to the differences in seasonal gas blends, it is not recommended that you stock up on cheaper winter fuel to save some money over summer. While using a winter blend in summer, or vice versa, won’t damage your car, it is likely to reduce performance and fuel efficiency. It will also produce more harmful emissions than using the correct formula for the season.

Via Opposite Lock / Mojo Motors

Photos by Kevin Dooley and Steven Depolo via Flickr