I have a 95 Acura Legend; I reset my trip odometer with every tank, so I know my mpg very well. I use premium gas (Chevron, if it's relevant.) Recently the station was out of premium so I used mid-grade gas; and lo and behold, it's getting better mileage. I've used it for several tanks now, and it's consistently at least 2 mpg better than with premium. Anyone have an explanation why that would be so?

The lower the octane rating number, the more volatile the fuel. Just make sure you are using the grade recommended by the manufacturer. That is very important. And use the premium grade at about every 5th tank. The premium grades usually have additives which help keep deposits from building up.