If you are wondering why your fuel economy has dropped recently, it is not because your car has something wrong with it, instead, it is because of the change that occurs every winter in the additives used in making gasoline. Specifically, the amount of butane added which is part of controlling the fuels “Reid Vapor Pressure” (RVP). During the summer months, the EPA mandates the RVP of fuel to be lower than that during the winter months to prevent the formation of ozone.
With older cars, the mechanical fuel pump designs made cars very susceptible to vapor lock in the summer months and flooding in the winter months, thus creating a need to adjust the fuel’s volatility. With the fine tuned fuel systems of today, that need has gone away. If we just ran summer blend fuels all throughout the year (giving us the best economy), no one would notice the difference. So the question remains, why do they switch to the winter blend fuels?
This is what the EPA has to say about it: There are some important differences between winter and summer gasoline. Most notably, winter grade gasoline has a higher Reid Vapor Pressure (RVP) than summer grade gasoline. RVP is a measure of a gasoline’s volatility, or the tendency for a gasoline to evaporate. Each spring, refiners must reduce gasoline RVP to comply with federal summer emissions requirements. Refiners, gasoline terminal facilities and retail stations must replace high RVP winter grade gasoline in storage tanks with lower RVP summer grade gasoline. EPA regulations stipulate that gasoline retailers must be selling only summer grade RFG by June 1 of each year. In order to meet the June 1 compliance date, EPA regulations stipulate that by May 1 terminals and all other facilities upstream of the retailer must have only those gasoline’s that meet the summertime requirements. [Source: EPA ] [Gasoline Volatility defined]
That leads me to suspect that the reason is about profits. The EPA most likely would like to see the summer blends run year round but were only able to mandate the stricter RVP guidelines for the summer months. When those months are gone, the petroleum industry is allowed to switch back to the fuel of choice which is cheaper to make, less efficient, more competitive, and in my opinion I suspect that this “Fuel of Choice” is more profitable as well. So until the EPA changes things, we are stuck with lower fuel economy from approximately mid September to some time in May, depending on how long it takes the gas station to empty their tanks.