91 but it's fresh , Renner Petro eureka sells high octane off road both leaded and unleaded , also race gas.
All Premium gas in California is 91 octane. It is possible to secure higher octane gas at various raceways, specialty stations, and via the use of additives. Costco complies with The federal Energy Policy Act of 2005 which required that a certain amount of ethanol be used in the nations' fuel supply. The Energy Independence and Security Act of 2007 increased the minimum amount of ethanol. Costco will also exceed the minimum amount of ethanol and at most stations provide E85 premium. Just look at the pump. If it says ethanol included then I believe the ethanol is 10%. If it says E85 then you will have 15%. I believe Costco Eureka has E85 but this doesn't reduce the octane below 91. In some rare cases, certain vehicles do not function well on E85. All modern vehicles should be fine.
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