In a manual transmission, direct drive is when the transmission is in its highest gear, and the input and output shafts are rotating at the same speed. This is achieved by engaging a dog clutch that directly connects the input and output shafts. Direct drive is also sometimes referred to as "fourth gear" or "fifth gear," depending on the number of gears in the transmission.
In an automatic transmission, direct drive is when the torque converter is locked up, and the input and output shafts are rotating at the same speed. This is achieved by engaging a lock-up clutch that connects the torque converter to the output shaft. Direct drive is typically engaged at higher speeds to improve fuel efficiency and reduce heat build-up in the transmission.
