"Today@ the motor vehiclemarket is focusing on ""lubed for life"" differentials requiring no service for the life of the vehicle. Still@ differentials are prone to develop problems of one sort or another since they are used to transmit a heavy torque through a right angle. Oneweak point in the differential is the ring and pinion gearset. As such@ a proper break--in period is essential to attain the required service life. Break--in is an attempt to smooth the contact surfaces of the gears and bearings through controlled or limited metal--to--metal contact. The roughness of the contact surfaces is reduced during this process until a lower and relatively stable surface roughness is reached. The lower surface roughness is advantageous@ but irreversible metallurgical and lubricant damage occurs since break--in always results in stress raisers@ metal debris and an extreme temperature spike. Break--in and its negative effects can be eliminated with chemically accelerated vibratory finishing. When this method is used to superfinish ground (AGMA Q10) or lapped (AGMA Q8) ring and pinion gearsets to less than 10 min.Ra@ the life of the lubricant@ bearings and gears is significantly increased. Just a few years ago@ this technology was considered impractical for high production volume OEM ring and pinion gearsets due to lengthy processing times. This superfinishing technology also had difficulties preserving the geometry of rough lapped gears@ which required more stock removal than finely ground aerospace gears (AGMA Q12+). As a result the transmission error of these gears was increased leading to unacceptable noise. The superfinishing technology discussed in this paper overcomes these obstacles and meets the needs of the motor vehicle industry. Gear metrology@ contact patterns@ transmission error and actual performance data for superfinished gearsets will be presented along with the superfinishing process."