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In the emissions tests on a chassis dynamometer, a measuring system equipped with a constant volume sampling system is used. The principle of its operation requires diluting of the raw exhaust with ambient air. To calculate the mass of the emitted pollutants it is necessary to know the extent to which the raw exhaust gas has been diluted. For its calculation, a dilution factor (DF) is used. The formula for this factor was derived assuming that the combustion is complete and that the fuel-air mixture in the engine has a stoichiometric composition. This article discusses the testing to verify the correctness of the assumption of the stoichiometric composition of the fuel-air mixture for vehicles fuelled with different fuels and tested in different cycles. The results of the measurements of the instantaneous fuel-air ratio for vehicles fuelled with petrol, diesel oil, liquefied petroleum gas (LPG) and natural gas (CNG) will be presented. Every vehicle was tested in the cycles used in approval tests: New European Driver Cycle (NEDC), World Light-Duty Transient Cycle (WLTC) and the ARTEMIS cycle, which is most often used to determine emissions in real traffic conditions. The instantaneous value of the air-fuel ratio was calculated using the Brettschneider equation based on the instantaneous concentration values measured in the raw exhaust gas.