Different techniques for measuring air flows are reviewed briefly. It is shown that the induction system geometry as influenced for example by a flow meter can have a significant effect on the lean mixture limit. Results are also presented that illustrate the transient performance of a viscous flowmeter with unsteady flow. Since it is difficult to measure the air flow accurately, and the measurements can alter the engine performance, it is argued here that the air fuel ratio should be computed from an exhaust gas analysis. The influence of the assumptions and measurement errors on the computed air flow, are investigated for different fuel compositions and air fuel ratios.