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My experience is that Octane number for premium grade gasoline depends on whether it is a 10% ethanol blend or not. Usually the non-ethanol premium is 91 or 92 and the ethanol blend is 93. The ethanol blend requires a higher octane rating because the ethanol makes it a less efficient fuel.
When I was in South Dakota last June, there were several stations that had their mid-grade 89 octane gas priced 10c/gal higher than premium 93 octane b/c the mid-grade did not have ethanol while the premium did.
DBear
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