"Winter Blend" Fuel is 2 - 8% Less Efficient? What?

There's a number of differences in winter blend gasoline, but there's really two major results of these changes that you're likely to note, and they're both caused by the same thing. The first thing most folks realize is that gasoline tends to be cheaper in the fall and winter months. Some of this is due to a drop in demand, but the truth is that the gasoline blend used in those months is actually cheaper to produce.

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Nick said:

Funny, I didn't know. I'm surprised the variance is that significant, up to 8%. That sounds very significant to me.

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Butane contains less energy per cubic foot than gasoline; the additional butane found in winter-blended gasoline results in a net energy loss of about 1.7 percent (114,500 BTUs per gallon for a summer blend, 112,500 BTU for winter blends). As such, using winter-blend gas will result in a maximum 1.7 percent reduction in fuel efficiency. However, bear in mind that the butane's additional volatility can hypothetically offset its energy deficit, allowing a cold engine to more efficiently combust its fuel. But since this additional volatility may or may not help a particular engine, it's better just to assume a maximum 1.7 percent loss.

Read more: http://www.ehow.com/info_12071245_low-gas-mileage-winter-due-blending.html#ixzz2iZfaacma
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Geoff said:

The drop in fuel economy is mostly due to the increased density of colder, dryer winter air. The engine will also be much less efficient until the oil warms up so a car that only goes a couple of miles in the winter and rarely warms up completely will get lousy fuel economy.

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