This article at The Oil Drum explains the backround and practical processes for the creation of the commodity known as "Summer gasoline".
Here in Texas, the Summer gasoline imperative is the need to keep emissions below EPA limits in 96 counties in North and East Texas. The EPA regulations require the use of what is known as "reformulated gasoline" in the Summer months. This has a double impact on what is available at filling stations:

1. The summer gasoline blend may give rise to starting difficulties if used in the Winter
2. The Summer blend has up to 10% ethanol added as part of the attempt to reduce the levels of emissions

For most pilots, (2) is the real issue, since ethanol in fuel attracts and absorbs water, which is corrosive to many fuel system components.
The overall situation is complicated by the fact that there is no legal requirement in Texas for gasoline outlets to report whether the fuel contains ethanol, or how much ethanol it contains. Some filling stations have stickers on their pumps saying "fuel contains ethanol" but many do not.
In Oregon, pilots gained an exemption from ethanol addition requirements for gasoline. The exemption applies to a limited number of outlets selling gasoline for aviation use only. No such exemption exists in Texas, so those of us who might want to try running our planes on road car fuel are SOL in the Summer because the fuel is almost certain to contain ethanol.