The base design from the original Torqueflite remains largely unchanged and the addition of a 2-speed output shaft (overdrive unit) that is bolted to the back of the 3-speed transmission has only two ratios: direct (1:1) and overdrive (.69:1). While lubrication to the overdrive unit was a challenge early on, this challenge was later overcome with factory improvements and/or aftermarket valve body kits. The overdrive planetary has six pinion gears (unlike the 5 pinion used with the A518 used with the Cummins turbodiesel) which is often used as an aftermarket replacement for the stock 4 pinion planetary used with the lighter duty transmissions.