I was listening to a news program the other day and they were talking about the Summer Blend of gas and the Winter blend. I really didn’t know what the deal was so I decided to do a little research.
First lets do a little learning –
Reid Vapor Pressure – (from now on we’ll call it RVP) – the scientific definition.
A common measure of the volatility of gasoline. It is defined as the absolute vapor pressure exerted by a liquid at 100 °F (37.8 °C) as determined by the test method ASTM-D-323. The test method applies to volatile crude oil and volatile nonviscous petroleum liquids, except liquified petroleum gases.
In laymans terms – RVP relates to the volatility of a gasoline. The more volatile a gasoline, the more likely it will evaporate as the temperatures rises; evaporated gasoline contributes to unhealthy ozone and smog levels.
So in the Summer time we want to have gas with a low RVP since it is less likely to evaporate. I don’t know if you’ve ever had gas out in a container (without a lid) during the Summer you’ll note that it evaporates pretty quickly.
The Winter blend has lower RVP. According to the EPA the Summer blend of gas has less energy, and is more expensive to produce. Some large cities and California (as well as much of New England) the EPA requires the use of RFG (reformulated).
Although the EPA says their is not enough difference to notice many people have noted that the summer blend gets much better gas mileage (some say as much as 4-6 miles per gallon).
Even though the winter blend is cheaper you won’t usually see a bump down in the price of gas.
The exact date of blend changes is a bit different every year – but a good estimate is in April the change to a Summer blend and September a change to the winter blend (again those are estimates).
Research for this article – http://en.wikipedia.org/wiki/Reid_Vapor_Pressure