Why is a gearbox needed?

Some vehicles use one gear, such as a Boss Hoss motorcycle, due to the low weight and large amount of torque. However, the average car does not have the power to weight ratio to achieve this.

A gear box allows ratiOS of motion between the engine speed and the wheel speed. As already mentioned, if you pick a mid range gear and only used that one gear, the car would be slow to accelerate and have a low top speed.

This is due to power delivery of engines. There is a hp and torque curve that are only at their peak during a relatively short range of rpm (revolutions per minute of the engine).

Theoretically, the more gears, the better the efficiency of the car overall.

try driving a car only in first gear and listen to the noise the engine would make then put it in gear 5 or 6 and see how hard it is to drive and how slow the acceleration would be and a gear box just allows the most efficient transfer of energy from the engine to the wheels in short