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Along the lines of Top Tier Gasolines, why do the same brands have different formulations in different parts of the country? I'm not sure if this is the case elsewhere, but in the U.S., I noticed in the mid-west the highest octane level was 93 but in the far west (Calif. & Nevada), high test is 91 octane. This would be a hint to me that the formulation is different for different markets. Does this mean Shell (for example) is Top Tier certified in all markets or just some markets? How does the % ethanol content affect the certification?
In my market area BP is the best. Why? They do not put ethanol in their fuel.

All Top Tier rated fuels are required to be minimum 10% ethanol. Ethanol produces worse mileage and power than 100% gasoline on an engine designed for 100% gasoline.

If you are using a boosted engine then the tuning can be modified to take advantage of the extra detonation control that ethanol has over gasoline. Otherwise it is not beneficial.

FWIW, I get about 1mpg better when I fill with BP over Shell, QT, Conoco, or Chevron. All are available in my local area.
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