I hate to tell you this, but your dad is right! You can overfill your tank—and doing so could harm your car’s evaporative emission control system (EVAP), which controls harmful emissions when gas evaporates.
You know that loud “thunk” you hear when you’re filling up, and it’s about done? That’s the pump telling you to stop pumping. Of course, it can be tempting to try to jam in a little more gas to hit that even number, but it’s so not worth it.
The EVAP is designed to work with gas vapors only. So when you overfill your gas tank, the liquid can cover the vapor intake hole and get sucked into the EVAP canister—damaging it and possibly other parts of the system.