In recent years, the focus on engine parasitic losses has increased as a result of the efforts to increase engine efficiency and reduce greenhouse gasses. The engine gear train, used to time the valve system and drive auxiliary loads, contributes to the overall engine parasitic losses. Anti-backlash gears are often used in engine gear trains to reduce gear rattle noise resulting from the torsional excitation of the gear train by the engine output torque. Friction between sliding surfaces at the gear tooth is a major source of power loss in gear trains. The effect of using anti-backlash gears on the gear friction power loss is not well known. As a part of the effort to reduce parasitic losses, the increase in friction power loss in the Cummins ISX 15 gear train due to the anti-backlash gear was quantitatively determined by modifying the methods given in ISO 14179-2 to fit the anti-backlash gear sub-assembly. A test case with a two gear mesh arrangement was designed for the purpose of validating the analysis by rig testing. The torque required to drive the test case as a function of gear rotational speed was measured on the rig. The test plan consisted of two configurations: 1. the anti-backlash gear and a regular spur gear mesh, and 2. the anti-backlash gear replaced by a regular spur gear. An anti-backlash and spur gear from the gear train of the Cummins ISX 15 engine - for the heavy duty market - were used in the rig. Results revealed that the anti-backlash gear-regular spur gear mesh required a mean torque of approximately 5 Nm, whereas the regular spur-regular spur gear mesh only required a mean torque of approximately 0.75 Nm. The power losses for these configurations were found to be in good agreement with the analytical results obtained by using the equation for friction losses according to Mauz. The conclusions of this study will impact engineering decisions that must consider the advantages of an anti-backlash gear (e.g. reduced gear rattle noise) with its disadvantages (e.g. cost and increased friction).