Auto questions: how gear ratios work

A gear ratio allows engines to operate optimally at different speeds. Detail about the differences in gear ratios and their parts and uses.

Whether your car uses an automatic or manual transmission, manufacturers have designed a specific set of varying gear ratios in the transmission. Some gears are better suited when moving off from a stop light or up a steep hill, while others work best when cruising along the highway. So, why are different gear ratios needed in cars and how do they work?

The majority of modern transmissions are designed with between four to six different gear ratios. These gear ratios are specifically chosen by the manufacturer for the particular engine or style of engine that your car or truck uses.

Despite the amazing power and torque that most modern engines produce, the internal pistons and moving parts will reach their maximum speeds at around five to seven thousand revolutions per minute (rpms). While this range of engine speed may seem very wide and impressive, in reality, it is a narrow range that wouldn’t allow our engines to be very useful in everyday driving without the help of gears of varying ratios.

Without the different gear ratios in our transmissions, engines would operate our cars with one gear only, which would have to be low enough to move the vehicle off from rest, yet high enough to allow cruising on the highway. If you put the shifter on a car in first gear, the car would move quickly away from rest, but at around thirty miles an hour, the engine would be screaming along near redline (manufacturer’s maximum ‘safe’ engine speed). Similarly, if the same shifter was put into a taller gear such as fourth, the engine would die and be unable to move the car off from a dead stop. However, if the car was already moving, the engine would probably start to come to life around thirty miles and hour, while highway cruising at sixty miles an hour would be at comfortable engine speed. While some motorized vehicles utilize one gear such as a golf cart or motorized scooter), their top speeds are not very high and their utility is generally quite limited.

This is where gears of varying ratios come into the picture.

The main purpose of a gear is to either magnify or reduce the speed of the output shaft (or crankshaft) from an engine. A shorter gear such as first or second on most transmissions, magnify our engine’s torque (twisting force) to the wheels by spinning slower than the engine’s output shaft. In this way, the engine is ‘assisted’ by the gears in moving a heavy object (our car) up a steep hill or away from a dead stop. Taller gears, such as those used in fourth or fifth gears, reduce the engine’s torque output to the wheels by spinning faster than the engine’s output shaft. This allows the car to move faster while the engine works at a leisurely pace – which in turn improves fuel consumption and driving quality (as engine noise will be low).

Our engines therefore use the different gear ratios in our transmissions to better utilize its power in all situations, improve both fuel economy and driving quality, as well as to keep the engine moving at a comfortable pace regardless of the vehicle’s speed.