i think its the car and its engine na talaga ang problem. The Optra is a gas guzzler talaga, changing to LPG doesn't change the fact that the engine dynamics are not in sync with the car. The car being a gas guzzler from the start doesn't pertain that you will get better mileage if you switch to LPG because your using the same engine. It will only mean that you are spending less money on the gas as compared to before but that wont change the fact that the Optra is still a gas guzzler.