[lifegetter]

Some octane trivia from the S.D. trip.

At home, and at most gas stops on the trip:
Regular unleaded = 87 octane, Mid grade = 89 octane

At Rapid City, South Dakota (just outside the Black Hills):
Regular Unleaded = 85 octane, Mid grade = 88 octane

At West Point, Nebraska (last fill up of trip)
Regular unleaded = 88 octane, Mid grade = 90 octane

(All Mid grade gas was enhanced with 10% ethanol.)

http://www.faqs.org/qa/qa-19161.html points out the reason for lower octane requirements in Colorado.