With the weather beginning to cool in the United States and Canada, the formulation of gasoline is also beginning to change. Regulations require gasoline with lower emissions and pollution in hotter months (summer gasoline), but those rules don't apply during the winter in many areas, so refiners move back to a type of gasoline that is cheaper to produce.

September 15 was the day that gasoline requirements were eased in many areas (aka the date of the first increase in RVP [Reid Vapor Pressure])- however- since the change to cheaper winter gas is not mandated, stations don't need to sell out of fuel before purchasing winter gasoline. Basically, since there is no requirement, stations don't purchase winter gasoline until they run low, so depending on a stations volume, you may not be pumping winter gasoline just yet. This is especially true premium gasoline- stations likely still have summer blended premium on hand and thus you may notice if you use premium that your fuel economy hasn't yet taken a hit.

Others may have already noticed a small drop in fuel economy. Winter gasoline contains more butane, which costs less, but also evaporates at a quicker rate and is more volatile. The savings that refiners see are passed on to motorists, similar to when motorists see prices rise as refiners produce summer gas each spring. Since the weather grows cooler, the increase in volatility is permissible.

Normal RVP values look like this:
January-March RVP is 13.5+ in many areas
April-September RVP is 7.0-9.0 in many areas
September-December RVP is 11+ in many areas

As temps warm, air pollution regulations mandate lower RVP numbers. The lower the RVP, the more expensive. Generally, a lower RVP will also indicate a slight increase in fuel economy.

To read on the change from winter to summer gasoline, check out this link to the EPA.

Want to see what type of fuel your area uses in the summer? This map is large and may take several minutes to download over a broadband connection- to see it, click here.

So who's noticed a drop in their fuel economy thus far?