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I am running 87 in the winter because 40 cent diff and I am in Upstate NY so snow. No need for more power. Once time hits for my Summer Tires it is back to 93.
Same thing I do...except its 60 cent diff in MI these days (I can remember when it was only 10). In the cold weather, I've noticed ZERO MPG difference. Not sure in the summer, since I only use 93 then.
 

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What mininsx said above is true. Summer fuel is mandated by the EPA, specifically to lower evaporative emissions. The main thing that refiners do with winter fuel is to raise the vapor pressure. This allows the fuel to vaporize more easily when cold, which helps engines to start more reliably. Try starting a carburated engine in 0* temps on summer fuel...it won't be easy. Winter fuel isn't any lesser "quality" than summer, but it does have a slightly different chemical makeup...primarily because more butane is used in the blend, which has a higher vapor pressure. It also has a lower BTU content, which results in slightly lower fuel economy. Cold startups, more idling, cold tires, increased drag due to higher air density, and lower BTU fuel all combine to decrease fuel economy in the winter.

If the pump says 93, it will be 93 octane fuel, all year long. Now, keep in mind that our octane rating system in North America is the (R+M)/2 method, which is different than most other countries, who go strictly by Research octane. (R+M)/2=AKI or Posted Octane Number. The Research or Motor octane numbers may move around a bit throughout the year, but the two still combine to generate 93 AKI/Posted octane. These slight changes could possibly cause your engine to be a bit more picky, but it shouldn't be significant.
 
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