This technical paper presents the results of full scale vehicle testing completed to examine deceleration on late model tractor semitrailer vehicles equipped with original equipment manufacturers (OEM) engine brakes. Testing consisted of measuring vehicle deceleration through a test section with the engine brake function both active and inactive. Also, rolling resistance measurements were taken to aid in the subject research project and also augment the body of available heavy truck deceleration data. Additionally, a comparison of the measured deceleration data to calculated values based on OEM supplied engine brake performance curves was completed. Results of the testing and research project indicate that theoretically calculated engine brake deceleration can be added to vehicle drive train resistance to arrive at an estimate of total vehicle deceleration. It is important, however, that a reasonable and realistic basis for rolling resistance and drive train resistance be established, as reliance on some currently published values could lead to over estimating deceleration.