The underbody of a truck is responsible for an appreciable portion of the vehicle’s aerodynamic drag, and thus its fuel consumption. A better understanding of the underbody aerodynamics could lead to designs that are more environmentally friendly. Unfortunately there are difficulties with correctly replicating the ground condition and rotating wheels when using the classical approach of a wind-tunnel for aerodynamic investigation. This in turn leads to computational modelling problems. A lack of experimental data for Computational Fluid Dynamics (CFD) validation means that the flow field in this area has seldom been investigated. There is thus very little information available for the optimisation and design of underbody aerodynamic devices. This paper investigates the use of a water-towing tank, which allows the establishment of the correct near-ground flow while permitting good optical access. Using a 1/10 scale model, Reynolds Numbers of around 0.7 million are achieved. A novel double light-sheet planar PIV setup is used to measure the flow field. This arrangement of two coplanar horizontal laser sheets provides data in regions that would typically be zones of no information with a single laser sheet arrangement. Two high-speed cameras are used, and their images are stitched together to give a large field of view at high resolution. It is found that this new experimental arrangement allows detailed quantitative flow data to be collected throughout the vehicle underbody. These results may then be used to provide a basis for the optimisation of underbody components.