I have been looking for a gas station for 87-91-93 Octane, but all the ones in Knoxville, TN have 87-89-93. I have started filling up with 5 gallons of 87 and the rest 93, so that on average I have slighly greater than 91 octane in the tank. One gal of 87 goes with about two gal.s of 93. If the pricing tiers are equal, it is cheaper to mix one 87 with two 93s than one 89 with one 93 (even though the math is easier )