The transmission on a semi truck has more gears than a car and a lower top speed; the low gear is around 2 MPH and the highest may be 64 MPH on one of the 16 speeds. If the driver is going down a hill sometimes they can drive over the top speed of the transmission by shifting to neutral and letting gravity do the work, which is called Kentucky Overdrive.
by Craigslist Mick November 4, 2023