if it was tuned for 91, you should run 91. same thing as if you tune for nitrous or MR12. its just not as an extreme a case and change as if you went from one of those two to regular pump 89. you would probably be just fine going from 89 to 91. personally, I just stick to 91. Its whats always been in the tank and through the motor, motor is used to running on it. 17k miles, why change it?
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