This paper examines the aerodynamic behaviour of plane-walled, single-plane-expansion, underbody diffusers fitted to a wind-tunnel model of a wheel-less, simple body having automobile proportions. The measurements were performed over a moving-belt assembly in the Pilot Wind Tunnel of the National Research Council of Canada (NRC). The purposes of the investigation were: to understand the governing physics of automotive underbody diffusers operating in ground proximity, to examine the effect of moving-ground and fixed-ground simulations on the behaviour of such diffusers and on the corresponding vehicle downforce and drag, to map the performance of simple, quasi-two-dimensional diffusers when used to produce downforce or drag reduction. The paper concentrates on the physics of the underbody flow in an attempt to understand diffuser behaviour in this application and its effect on downforce and drag, and to gain new insight into the relative characteristics of different vehicle ground simulations.The data are analysed from the diffuser perspective by relating the measurements to the conventional diffuser parameters of area ratio, non-dimensional length and flow-area blockage. This last parameter, which represents the velocity non-uniformity in the inlet flow due to boundary layers on the underside of the model and on the tunnel floor, is one that may contribute to diffuser performance differences between the fixed-ground and the moving-ground simulations.A performance characteristic defining diffuser geometries for maximum downforce has been identified. Knowledge of its functional behaviour offers the possibility of developing similar curves for new vehicle geometries with a reduced number of measurements or computations.The data acquired demonstrate that an underbody diffuser can significantly increase downforce, and can provide some drag reduction if ride height is not too small.