Gearbox
The gearbox lets the engine run at an efficient speed while the wheels turn at the speed you need - its job is maintaining the desired speed for the conditions.
Low gears give pulling power: use first to move off, on a steep climb, and to hold the vehicle back on a steep descent. High gears give economy on an open road.
Change up as the engine's note rises and down before it labours. Changing down to slow the vehicle saves the brakes; changing up too early makes the engine work harder for the same speed.
Power reaches the wheels through the DIFFERENTIAL, which lets the driven wheels turn at different speeds through a corner.