What device is added to transmissions to equalize the speed of the mating parts before they engage?

The correct answer is the synchronizer. A synchronizer is a crucial component in a manual transmission system that allows the gears to engage smoothly by equalizing the speeds of the mating parts before they come into contact. This process minimizes grinding and helps ensure that the gears mesh without resistance, improving the overall performance and longevity of the transmission.

When a driver shifts gears, the synchronizer works to match the rotational speeds of the engaged gear and the gear being shifted into. This is achieved through a series of mechanisms, such as friction cones, which help bring the speeds into alignment. If the speeds were not equalized, it could lead to difficult shifting, excessive wear on the transmission components, or even damage.

Other options relate to different functions within a vehicle's transmission system. The clutch, for example, serves to engage and disengage the engine from the transmission, allowing the vehicle to change gears. The shift lever is simply the mechanism used by the driver to select the desired gear. The governor, meanwhile, regulates the engine speed by controlling fuel flow, which is a different function entirely. Therefore, while these components each play essential roles in vehicle operation, it is the synchronizer that specifically addresses the need for equalizing speeds during gear engagement.