Fill your tank with half 93 and half 89 = 182 divided by two equals 91! It's my understanding that most stations actually create their 89 (Plus) by mixing the correct ratio of 93 & 89. I'm no expert, but can't using too high of an octane also cause problems? The octane number doesn't necessarily mean that it's "better", just that it's formulated to burn at a higher temperature.