Among the different wireless solutions, ultra-wide band (UWB) is a promising technology for in-car communications because of its high data rates. To optimise the UWB radio system design, knowledge of the propagation channel within the car is required. The performance of a high-speed 480 Mbps UWB radio system is studied within a real in-car environment measured under mobility. A comprehensive set of measurements is presented, including several possible non-line of sight scenarios while the vehicle is stationary and mobile, for open- and closed-window environments and with/without occupants. These measurements are used to characterise the in-car channel and evaluate the performance of a typical UWB radio system in this setting.