Yes, miles driven and gas bought at fill up. Granted the fraction at the end of the fill up won't be identical every time but not enough of a difference to account for 2/3 mpg for the whole tank-full. I'm not really complaining as the mileage is considerably better than my '99 Forester, just disappointed.I assume you are calculating this based upon odometer readings and amount of gas when you fill it up. I'm not saying you are incorrect in what you are saying but calculations are all dependent upon miles driven being accurate (which depends upon tire pressure and temp) and gas stations also accurately measuring what the sell you.
Advertised gas mileage is also based upon ideal driving conditions etc.
I guess to me being 1-2 mpg off doesn't surprise me.
According to the EPA website:Not really because what it said I was getting was more than the car is advertised to get and I know they would never understate the mileage in advertising.