I agree with most here... however, my 92 Legacy L FWD MT used to get ~32 mpg with highway driving. Kept car well tuned, no aggressive driving, etc. When I would drive at higher elevations I always got better mileage-- colder air maybe? Even then the most I would get is ~34mpg.
In my experience though, as others have stated is important, those numbers were consistent over 40-50 tanks of gas. I didn't just get those numbers for a tank here and there-- I recorded every single fill up and averaged them at the end of the year.
Nevertheless, you very well could have gotten ~39mpg for that one tank, but take it for an anomaly-- not something to be depended on.
Fill up 3-4 times more times, and add total miles traveled over all tanks, including your 275 mile 7 gallon trip, and then divide by the total gallons used over all tanks for those miles, plus your 7 gallons from your trip.
After that tell us what your numbers are, and most here will be more apt to give credence to the results.
Even if you do a full tune-up, or anything else in there, over 3-5 tanks of gas you are going to see numbers that are more reflective of oem specs and/or general user experience.
But, I would be excited about a higher than normal mileage out of a tank of gas too.