The various methods for calculating AFR from exhaust gas analysis do not always produce identical results. This paper presents exhaust gas analysis data from a four-stroke SI engine burning two different fuels - unleaded gasoline and LPG. Exhaust gas concentrations are measured both upstream and downstream of the three-way catalyst. All four sets of data are analysed using established algorithms due to Spindt, Urban, Uyehara and Fukui. The results of each algorithm are compared with the AFR obtained using direct measurement of air and fuel mass flows.