A link to another discussion of octane. Most has already been said.
http://www.ftc.gov/bcp/conline/pubs/autos /octane.htm
Utah is one of a few high altitute states that can use 85 octane in any cars. It causes pinging and knocking at other altitudes and isn't sold in most states. 87 is the lowest for most states.
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Cougars'Paw