"When you hit the blinking pip, you still have about 2.0 gallons left. I added a touch over 20.6 gallons and if the Sonata tank is the same as the Optima's, it will hold 22.2 gallons or thereabouts.
A caveat to the hybrid however. If you were to run it out, the traction battery will continue to spin the ICE over trying to start it up until the big pack is completely depleted and possibly damaged. I have not verified this but I spoke with a tech about it last year and he passed along this "Never Do This" warning."
CleanMPG Forums - An All-American Road Trip with the 2012 Hyundai Sonata Hybrid