2013 Hyundai Sonata Decrease in fuel tank capacity

Scan for codes and check fuel pressure then check to see if any of the brakes are dragging or low tire pressure, alignment etc.

I'm just curious, are you strictly going by what the dash is telling you how many miles are left until empty or are you calculating it on your own for each full tank of gas. If you're calculating it on your own, you would need to write down how many miles you physically drove on each tank of gas and then average it out. Just remember, this is just an average because no full tank of gas will ever be exactly the same due to driving habits, weather, etc. If you're going strictly based on what the dashboard is telling you, this is never accurate for some reason and always seems to change. You can have a reading of 360 miles, disconnect the battery for 10 minutes until everything resets on the vehicle, reconnect the battery and then check the reading, and I can almost guarantee you that the number changed even though you did not even move the car. I work for a hyundai dealership and we've had multiple complaints about it and as a technician, I've contacted the Hyundai Techline for explanations and the explanation I'm giving you is the only one we ever get unfortunately. They tell us its just a rough estimate. I personally would just prefer a needle telling me where the gauge is at.