Excellent car with average mileage

Everything is excellent whether you talk about interior, exterior, premium feelings, comfort, power only except mileage. In city drive with CNG I get 18 KM/KG & on highway driving I get around 24/25 km per kg. Mileage is what I feel too low in comparison to other CNG cars. Otherwise this is the only sedan car available in the market with CNG and gives a premium feeling too. Service cost also minimal.