I would suspect that a number of other factors influenced the high mileage of this particular tank--prevailing winds and travel direction, temperatures, number of acceleration/deceleration events, rate of acceleration events, average speed--any number of things. You can't chalk it up to the premium right off the bat without controlling for other variables. Here's what
Cartalk has to say:
"Premium gas gives you more miles per gallon than regular gas.
Sorry, that's pretty much booo-gus.
Here's why.
Each gallon of gas that we pump from our local Quickie Mart is actually made up from as many as seven different ingredients. The exact amount of energy in each gallon of premium or regular gas will vary from company to company, depending upon what kind of additives they use.
In actual fact, you'll get a greater range of fuel economy between different brands of regular gas, than you will between the same manufacturer's regular and premium gasses. Interesting, eh? We thought so.
Finally, here's a nice irony: to increase gas' octane rating, companies add ethanol, when they're mixing up a batch of premium fuel. Interestingly, ethanol actually contains less energy than untreated gas, so the net result from the ethanol component is a reduction in your MPG. Other premium additives, however, have the reverse effect, and slightly increase your MPG. So okay, overall premium provides a very slight net increase in MPG, but it's so slight that we swear you won't notice the difference.
I always used the increased MPG that resulted from using premium as a justification to purchase it.
Too bad. Consider yourself better informed now, and stop lining the pockets of oil companies, okay?"