Here is the jist of what happened.

-Friday night fill up tank, There was about 1/2 tank in the car already.
-Gauge reads full when leaving gas station.
-Reset mileage counter
-Drive 280km one way. Gauge reads still over 3/4 full on arrival destination (never ever have I got that good gas mileage)
-Drive car around on weekend a little
-Sunday I fill up the car
-Gauge shows full after filling up.
-Reset mileage counter
-Drive home (another 280KM)
-Gauge reads still over 3/4 full

The gas gauge seems to be working. When it was full it read full. But for that distance I should have used almost 1/2 tank as I usually have on that same trip. (make it every couple months)

So I need little help from the experts here. Could the sending unit be going bad that reads the fuel level or float in the tank? When the car is off the gauge reads empty. When I start the car it goes up as normal.

Any help would be appriciated.