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it works if and only if your car is designed to run on high octane fuel (like the 944) - it's not that your car will get better mileage than what is specced with 91 if you go higher - it is that your car will get WORSE mileage than specced with the lower grade fuel
just like power ratings, fuel economy ratings are based on the highest fuel specced for the car - if the car was rated at 26mpg, it is nearly impossible to get than on less than the higher of the specified fuels (in the case of the 944, that would be 92) - in the event of running 87 on a car that got 26 on 92, you would then get about 23.4 or less
try it out - do the math - see if it then makes sense for you
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