I have the same problem with my City 2009. I think it is common with most of City user due to computer miscalculation of the fuel use while on CNG (City does not come with factory fitted CNG). I do the following to be ascertained of the Petrol level in my Car:
1) Get some petrol filled in my tank even one or two liter will serve the purpose by bringing the petrol gauge needle back to actual.
2) Unplug and again fix a terminal of my battery, so computer is restarted and shows the actual Petrol gauge reading.
I hope it will clarify <label for="rb_iconid_7"></label>