After the light came on, I drove 20 kilometers and then refueled 65 liters. I'm starting to doubt if my fuel tank is actually 80 liters.

The capacity is 80 liters, and when the light turns on, there's about 20 liters left. After the light comes on, you can still drive for about 60-90 kilometers. Running 20 kilometers consumes about 2.5 liters of fuel, so there's about 63 liters left. However, the bottom of the fuel tank isn't flat, and the fuel pump can't extract all the fuel. You can try driving 10 more kilometers after the range display shows 0 before refueling.