You may notice that your gas mileage has gone down a little bit in the past couple of months. Before we go blaming on ethanol (even though ethanol’s been in the fuel all year), you might want to consider the little-known fact that “summer blend” gas is actually different from “winter blend” gas.
What’s this? Gas isn’t the same all year round? Well no, it really isn’t.
Gasoline formulators vary the composition of gas in the hot summer months and the colder winter months, in order to meet emissions and air quality standards. This “reformulated” gas is blended in such a way that it has a lower vapor pressure in the summer and a high vapor pressure in the winter blends. This means the summer gas evaporates less and thus contributes less to the “evaporative emissions” which represent million of gallons of gasoline per year evaporated in the air. Winter gas is more volatile and evaporates more quickly, so it is ideal for it is used in the colder air of winter.