Like other developing countries (Brazil, Argentina, and India) compressed natural gas (CNG) is becoming a popular vehicular fuel in Pakistan. Rapid shifting of diesel and gasoline vehicles to CNG has brought Pakistan the highest number of CNG vehicles in the world. To quantify a possible decrease in vehicular emissions for different types of vehicles, engine and fuel types were monitored for five parameters: SO₂, CO, NO, hydrocarbons, and smoke opacity. Emissions from heavy vehicle engines shifting from diesel to CNG showed a decrease in HC (14 times), NO (2.8 times), and smoke opacity (3.2 times), while shifting diesel car engines to CNG resulted in reduced emissions of HC (24.6 times), NO (2.8 times), and smoke opacity (6 times). However, switching of light vehicles such as gasoline car engines to CNG released low emissions of HC (4.6 times), smoke opacity (1.2 times), SO₂ (1.2 times), and CO (1.1 times), but an increase in NO (1.2 times) was observed. Similarly, a 4-stroke CNG rickshaw engine increased NO emissions by 1.4 times over a 4-stroke gasoline rickshaw engine.