I visited Toyota Dealer and had discussions with their chief mechanic about CNG conversion in Vitz/ Platz and he categorically told me that CNG is not feasible in Vitz, Platz & Altis due to Oxygen sensors problems etc which are only there for environment issues. However, the O2 sensor will give u better mileage due to optimum level of air/fuel ratio all the time. Instead, he told me to drive these vehicles on un-leaded petrol and u'll get a mileage near to CNG.