Problems such as global warming and rising oil prices are driving the implementation of ideas to reduce liquid fuel consumption and greenhouse gas emissions. One of them is the use of natural gas as an energy source. It is a hydrocarbon fuel with properties that allow the reduction of CO2 (Carbon Dioxide) emissions during combustion. Solutions are being implemented that allow for the use of natural gas to means of transport, namely in trucks of various categories and intended use. These installations are used in new vehicles, but also in the form of conversion for used cars, usually several years old. The article presents the results of tests of an engine from a used semi-trailer truck with a mileage of approx. 800 thousand km, with the compressed natural gas supply system installed. This installation (hardware and software), depending on the engine operating conditions, enables the replacement of up to 80% of diesel (base fuel) with natural gas. The impact of changing the fuel supply method on the traction characteristics calculated with the use of external characteristics of both conventional and dual-fuel mode was assessed. The emissions of exhaust gas components were also determined under the conditions of the UNECE Regulation No. 49. The test results confirm that compared to conventional fueling, dual fueling allows for a significant reduction in CO2 emissions, even in a used vehicle with high mileage. The use of a non-factory installation did not significantly affect the traction properties of the vehicle, and engine wear is of greater importance in this case (comparison with factory data). The work is a valuable supplement to the results of the research in which the impact of the use of a non-factory CNG (Compressed Natural Gas) supply system on the performance of a semi-trailer truck unit equipped with such an installation was assessed compared to a semi-trailer truck unit powered in a classic way with diesel fuel.