its good
because burning fuel is a chemical reaction. The gases produced from this reaction are C0 (Carbon Monooxide), CO2(Carbon Dioxide), NO (Nitrous Oxide), H2O(water as steam). So as the steam flows out of the exaust it condenses because of the cold temprature outside. So the more efficient your engine is, the more H20 and C02 from your car and lesser CO and NO. CO2 is a green house case, CO and NO are pollutants.