A gearing setup that provides a ratio of less than 1:1, meaning the driven gear rotates more times than the driving gear per revolution. This reduces the mechanical advantage and torque output but significantly increases the top speed of the output shaft. It is heavily used in automotive transmissions to lower engine RPM at highway speeds for better fuel efficiency.
Heard on the job
When the gearbox shifts into overdrive, the engine RPM drops right down to a hum.