The increasing importance of drag reduction in commercial vehicles, traditionally motivated by rising energy costs, and more recently accelerated through policymaking in a bid to reduce emissions, has resulted in the development of increasingly sophisticated add-on aerodynamic devices for the semi-trailer. Of these devices, the greatest uptake amongst trucking fleets has been for devices fitted to the underbody, primarily on account of allowing free movement of freight into and out of the trailer, hence incentivizing a better understanding of the flowfield in this region. The current research addresses the aerodynamics of trailer wheels in tandem, which form a key component of the semi-trailer underbody, by investigating the effects of wheel rotation, trailer side-skirts, and wheel cavity covers on the flowfield and drag. The configurations tested include a short side-skirt terminating before the front edge of the leading wheel, a long side-skirt covering both the wheels, wheel covers without openings (solid disks), and wheel covers with openings varying in coverage area and radial position. Stereoscopic (2D-3C) PIV is applied to examine the flow topology in the near wake and on selected planes besides the wheels. The measured velocity data in the wake is further used to derive the pressure field (by solving the Poisson equation for pressure), which together are then used to calculate the drag using a control volume approach. The uncertainty in the derived pressure field and drag values is determined using linear uncertainty propagation. The largest drag reduction from the baseline case, i.e. without a side-skirt or wheel covers, is seen for the long side-skirt, followed by the short side-skirt with wheel covers, short side-skirt only, and wheel covers only, in that order. The effectiveness of both the long side-skirt and wheel covers is seen to increase with wheel rotation, with an appreciable reduction in drag with the wheel covers fitted only seen for rotating wheels. Investigation of the flowfield in the wake shows significant differences between the stationary and rotating wheel, particularly within the region of the projected wheel profile, and indicates to an earlier separation of flow along the upper surface of the rotating wheel. The effect of the side-skirt on the velocity in the wake is seen primarily in a lower streamwise velocity deficit, narrower wake, and higher horizontal symmetry of the wake for the skirted configuration. Wheel covers show a comparatively limited effect on the velocity field in the wake, showing a marginally wider wake and a slightly higher velocity deficit for the uncovered wheel. On planes beside the wheels, the side-skirt and wheel covers show a greater influence than wheel rotation, considered to be a consequence of a static floor combined with a gap between the wheel and the ground. The non-skirted configurations here exhibit a larger region of separated flow and greater velocity deficit whereas the uncovered wheels show an outflow at the bottom of the wheel cavity and an inflow at the top. Finally, the wheel covers with openings show a behavior between that of a covered and an uncovered wheel, with the inflow/outflow depending significantly on the coverage percentage and radial position.