I have subjectively experienced the benefits of premium fuel in several vehicles that I've owned. I have also subjectively experienced one brand of fuel that always seemed to leave the car feeling "flat".
However, objectively none of my data (I have recorded fuel quantity and mileage for every drop of fossil fuel I've put into my own vehicles) supports any benefit or any negative between the different fuel suppliers so far as mpg goes. Of course there are differences, but they pale into insignificance against the statistical errors that exist in the data (eg one pump cuts off sooner than another, bad weather vs good weather, feeling like "having a drive", etc).
I haven't yet given the Leaf a try on Premium. Will report back.
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