The DMC-12 manual transmission is a constant-mesh five-speed. All forward gears are fully synchronized for ease of shifting. The aluminum case consists of two halves which, when repair is necessary, can be separated to provide easy access to internal parts. The final drive components are contained within the transmission case. Both the final drive and transmission use the same lubricant.

All gears are selected manually by the driver using the gearshift lever attached to the selector lever mounted on the side of the transmission rear housing. Inside the rear housing, a selector shaft arm, attached to the selector shaft, fits through a slot in the pivoting interlock. When a gear is selected, the selector shaft arm moves the interlock to the desired position and moves the appropriate shift rod in or out depending upon the gear selected. The shift fork attached to the shift rod moved the desired gear into the proper position while the synchronizer ring matches the soeed of the two gears being coupled. This shift sequence is repeated for all forward gears. Reverse operation is the same with the exception of the synchronizer ring, which is not required, since the vehicle should be stationary when shifting to reverse.