why is it every time I fill up my 2013 Sonata it says less gas millage to the gallon

Try filling up your car next time when it's cooler at night. It actually makes a difference as to when you refuel. Gasoline expands with heat. Try it and let us know.

If you do not reset the fuel data unit each time you fill up, it is measuring the results over the time period since have done it last. It would be typical to have a higher estimated mileage when you first filled it up than what it is registering now. The current number represents more driving conditions over a greater number of miles. As "Tennisshoes" said, to get an accurate reading, measure it yourself over about a month of typical driving being careful to fill it the same way each time during the period (don't just stop pumping when the nozzel shuts off unless you use the same pump every time).

Yes, you need to reset the fuel trip data each time you refill your tank. Here's a simple explanation why: say you're driving about town for the majority of one tank. The estimate on number of miles to refill will be based on your MPG ( stop & go driving) . Perhaps your next trip is completely highway miles. If you don't clear all the trip meters from the previous fill, BOTH of your trips will be averaged together, and fill time will be inaccurate. That's if you care about, or are dependent upon the guage that says "how many miles until refill". You can ignore this guage if you like and just pay attention to the one which has the meter on it, showing the status between empty and full, just like in the olden days. The newer meters just keep averaging all your trips together, unless you clear them. I like to do this so that I can tell if my fuel usage is changing, signifying a possible performance problem. Another, more personal example which I like to cite is, if you didn't clear the trip meters after divorcing your spouse who drove like a maniac, the car would STILL be averaging in his a**hole 21 MPG with YOUR more sensible 30 MPG. - Guess who got custody of the car??