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the car's computer measures mpg based on odometer and injector duty cycles over time.
I'd consider it far more accurate than the implication that you're able to 100% duplicate the exact driving conditions day after day so that the engine uses exactly the same amount of fuel each time.
Further, fuel octane doesn't change mpg. Fuel density can, and in general, higher octane fuel is regular fuel mixed with alcohol to inflate the octane ratings and alcohol has a lower energy density than gasoline, but the addition of alcohol is not guaranteed in a given grade of fuel, 93 etc existed before alcohol started being added wide-spread in most fuel. So that will vary state to state and even supplier.
My mid 40mpg 50 mile span runs (measured both ways ) were done with 91. The amount of alcohol difference in 87 vs 91/93 is going to be insignificant in terms of overall energy density. You get a bigger difference between winter and summer blended fuels and even those are on the order of maybe 10% less mileage.
The primary impact octane will have is in timing. 91/93 will let the engine use it's more aggressive timing tables and retain advanced timing over a wider rpm band than lower octane fuel will. How often you make use of that aggressive timing determines how much "feeling" you get when determining if there's a difference or not between the two. If you're heavy on the acceleration all the time, you'll feel a difference (and avg very low mpg - obviously) ...if you're light on it, you'll never notice a difference.
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