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When reconstructing a highway accident involving a heavy truck, an accurate deceleration rate is often an elusive piece of the puzzle. This paper presents a method for calculating the deceleration rate of a heavy truck equipped with an air-mechanical brake system. Traditionally a deceleration rate had been more an estimate of the percent of wheel lock-up based on the brake adjustment, and then proportioned to the loading on the axles. Equations for two configurations of brake chamber size and slack adjuster length were written by engineers at the National Highway Traffic Safety Administration's (NHTSA) Vehicle Research and Test Center (VRTC) to describe braking torque as it varies with pushrod stroke. These equations were developed by means of a regression analysis, to fit a curve to dynamometer data produced at the VRTC in East Liberty, Ohio. Once this braking torque is converted to a braking force, it can be compared to the braking force available at the tire/road interface.