Not satisfied with Mazda 3 fuel capacity that I bough

I went to Mazda Fury Midrand and purchased Mazda 3 2017 model which I am not satisfied with fuel capacity/consumption and I am stuck with this vehicle for 5 years and I can't take it. The tank of the vehicle is too small everytime I fill up my tank it gives me low kilometers range NB 457km on a full tank of 51 litres which is not correct. I called the dealer and told them my problem and they said they will get back to me even now they haven't. It is very sad to experience this problem when South Africa is facing financial problems, I need assistance on this.