Example embodiments relate to measuring rotational speeds of trailer wheels using radar. A computing device may cause a vehicle radar unit to transmit radar signals toward a wheel of trailer being pulled by the vehicle. The computing device may receive radar reflections corresponding to radar signals that reflected off the wheel and determine a rotational speed of the wheel based on the radar reflections. For instance, the computing device may identify the highest or lowest frequency in the frequency spectrum of the radar reflections and use the frequency and the wheel's radius to calculate the rotational speed of the wheel. The computing device can use rotational speed measurements for trailer wheels to monitor performance of the trailer and adjust vehicle navigation accordingly. In some instances, the computing device may determine that one of the trailer wheels requires servicing based on monitoring the rotational speeds of the trailer wheels.