The problem of global warming and the related climate change requires solutions to reduce greenhouse gas emissions, in particular CO₂. As a result, newly manufactured cars consume less fuel and emit lower amounts of CO₂. In terms of exhaust emissions and fuel consumption, old cars are significantly inferior to the more recent models. In Poland, for instance, the average age of passenger cars is approximately 13 years. Therefore, apart from developing new solutions in the cars produced today, it is important to focus on measures that enable the reduction in CO₂ emissions in older vehicles. These methods include the adaptation of used cars to run on gaseous fuels. Natural gas is a hydrocarbon fuel that is particularly preferred in terms of CO₂ emissions. The article presents the results of research of carbon oxides emission (CO, CO₂) in the exhaust gas of a passenger car fueled by petrol and natural gas. The emissions were measured under the conditions of the New European Driving Cycle (NEDC) test and in real road tests. The test results confirm that compared to petrol, a CNG vehicle allows for a significant reduction in CO₂ and CO emissions in a car that is several years old, especially in urban traffic conditions.