Also, you can't gauge a fuel mileage increase off of one single instance. And the things jetrep mentioned above. You'll need to make multiple trips, fuel logging, and you'll also need to take the SAME trips repeatedly to get a mean and an average. You'll also need to record weather conditions for the day.
I can probably get 41MPG in my N-Line, if it's a calm day, flat land, use a light touch on acceleration, and keep it under 50MPH. I was able to get 52MPG in my 2017 Elantra Value Edition sedan (rated 39MPG highway) once, but in that instance I DID have a tailwind, and I was doing between 45 and 55MPH with the air conditioner off.