This paper proposed a method for estimating the suspension relative velocity using a single six-dimensional measurement unit attached to a vehicle body. This method can reduce the number of sensors, reduce costs, and address sensor packaging issues. A wheelbase preview assumption is used to overcome the lack of unsprung mass information. The proposed estimation method is verified by the vehicle simulator Carsim and the stability of the estimation method is also analyzed. In addition, the frequency response and the time response of the sky-hook controller based on the estimated relative velocity are compared with the sky-hook controller based on the actual relative velocity. The results show that the proposed estimator produces very robust performance on various road conditions.