for a GT you are fine running a 89 or even an 87 for that matter. i'd use the 89, ethanol is good for your engine. using a 91 would just be a waste, wouldn't do anything for your engine
91 should be all you need. 93 or above would be better and maybe overkill, but 91 for your GT you should have no problem or complaints even lower. It's safe and reliably. If you hear your engine ping or knock then upgrade to a better fuel. I run 100 + in my GTS. Only because I notice a difference in the octane, I don't drive that often or far in my Celica, and I can afford it. 100-108 in my car no matter what any body says HELPS my acceleration and trap speed amongst other reasons.
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