Daily Monitor Summary A slightly bigger 1.8 litre engine may prove to be more efficient when driving in performance demanding situations such as the highway Advertisement Which petrol engine is more efficient, 1.5L or 1.8L? Hello Paul, I am looking to buy a car but I do not know which engine between 1.5cc or a 1.8cc is more efficient. I am looking at the Toyota ISIS 1.8cc, Honda Airwave 1.5cc and Rumion 1.5cc. Francis Hello Francis, the choice of engine size is usually driven by fuel economy, performance needs or purpose of the vehicle. In this case, you seem to be looking for a fuel sipping engine with reasonable performance, a measure of efficiency. Ordinarily, the smaller 1.5litre petrol engine should give you a superior fuel economy of about 16-18 kilometre per litre while the bigger 1.8litre engine will give you about 14-16 kilometre per litre.