For the GTP guys run the highest you can get period. I know around here 93 is about .10 more then 89 (no 91 here) which on a tank of gas is just over a doller more. Yes I know when you add the tanks up it gets pretty high but do your car a favor and give it what it requires. People have actually experienced worse fuel mileage bacause of the octane too low and that means buying more fuel more oten.