Abstract: An apparatus and method for measuring the circularity of a brake drum while the brake drum is mounted upon a vehicle. The apparatus includes a linear voltage displacement transducer mounted upon a stationary portion of the vehicle. The transducer senses the variation of the braking surface and generates a signal proportional thereto. The apparatus also includes an optical encoder that generates a signal indicative of the angular position of the drum. The transducer and encoder are connected to a central processing unit which receives the transducer and encoder signals. The drum is rotated through a plurality of complete revolutions while the central processing unit combines the transducer and encoder signals to generate a set of data points. The central processing unit operates upon the data points to determine the circularity of the braking surface.