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.
In terms of gas mileage, you get better mileage on summer blends than winter blends because the summer blend gasoline has about 2% greater energy value than winter blend. So the gas you are using now most likely has a lower energy value than the gas you used in the winter. That means a little bit lower mileage for you until the spring time, when they will reintroduce summer blend gasoline. And this is beyond mileage loss you see with ethanol being blended into both summer and winter fuels. Cars, trucks, boats and small engines, all will see the difference in mileage and fuel consumption between the seasons.
Why can’t they just use summer blend gas all year round? Summer blend gas doesn’t work as well in the cold winter months because it is less volatile and doesn’t evaporate as well. This contributes to starting problems in the winter. Hence winter blend gas is prepared to eliminate this issue.
Can you avoid the summer gas/winter gas switch? Not likely if you live in California, New England or a major city of any appreciable size.
Watch a few video testimonials of happy Bell Performance customers who have enjoyed better fuel mileage while using Mix-I-Go and other products.
This post was published on December 2, 2010 and was updated on March 29, 2018.