The petrol gauge in my Mazda 6 has been unreliable ever since I bought it, so much so that I have run out of fuel when the gauge said ? full and the trip computer said I had 120 km to empty. My Mazda dealer told me it was common for the gauge to be unreliable on Mazda's, and there was nothing that can be done. I now rely on my trip meter to fill-up after 600 km of driving.
There is something wrong with the fuel gauge, so don't be fobbed off by the dealer. Insist it be fixed, they could always replace the gauge and any other parts that are faulty in the system.