I've seen add-on overdrive units for RWD vehicles, I believe one such unit ("the ranger" ?) fits between the clutch and main gearbox on New Venture transmissions. There are a few others which divorced-mount behind the transmission, or in some cases behind the transfer case on 4x4's whose owners need to remember which range they're in before engaging the front axle
No such unit would be useable on any type of FWD or AWD car.