The Marathon 600-21 is a six-speed transmission built by the Marathon Automobiles Company in the early 1970s. It is an all-synchronized, overdrive transmission designed for the heavy-duty, commercial truck market. It features sturdy construction, full synchronization between all six speeds, and is designed for strength and durability. It requires a separate synchronizer assembly for all six speeds, and is equipped with a mechanical governor. The Marathon 600-21 has a relatively low-ratio, 1st gear of 8.12:1 and trims up to a final drive of 1:1. It is a reliable and dependable transmission, making it a popular choice for many commercial trucks.