Why is gas more expensive in the summer? Competition, logistics — and the blend

As summer approaches, Canadian drivers have likely already experienced the all-too-familiar rise in prices at gas station pumps.

A boosted demand for gasoline by people on road trips affects prices, as did the latest increase in the federal carbon tax in most provinces and territories. 

But there's another key factor that might not be as obvious. And just like a good Canadian whisky, the secret is all in the blend.

Winter gas contains butane, a relatively cheap additive that helps cars start faster and run better in colder weather. In the summer, butane is left out in favour of alkylates, which burn cleaner but are more expensive to produce.

"Essentially it's to limit air pollution during the summer months when many Canadians are on the road travelling," Patrick de Haan, head of petroleum analysis at GasBuddy, told CBC Radio's Cost of Living.

GasBuddy is a popular app that crowdsources gas prices in Canada and the U.S.

De Haan said the additional cost to consumers of switching to summer gas is around 10 to 15 cents per litre. However, the hike could be more or less depending on the precise blend used in a particular year or region. 

The switch in composition is mandated in the U.S. by the Clean Air Act, which among other things dictates that gasoline sold in the summer must have a lower Reid vapour pressure (RVP) number. That number is essentially a measure of how quickly the gas evaporates when burned as fuel.

Each province and territory has different RVP minimums and maximums as set by the Canadian General Standards Board. While they vary by location, they're all higher in the winter months but lower between April 15 and the end of August.

Environment and Climate Change Canada did not respond before publication to a CBC request for additional information. 

The changeover from winter to summer gas blends in Canada and the U.S. are generally in sync, since refineries in one country can serve both markets, de Haan said. 

Butane's high RVP helps a car start faster in the cold, but burns fuel faster than needed in hotter summer months, contributing to more carbon emissions.

"When temperatures are very warm, even the roads can dry very quickly after a rain," de Haan said. "That's why it's important that we make this transition [so] that we have a gasoline that is more resistant to vaporizing."

What's in the gas isn't the only thing that matters. The logistics of switching from one type of gasoline to another puts further pressure on prices.