# How do the gears on a car work and what do they do?

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My Dads tried telling me loads of times but I really don’t understand them.

In: 5

The transmission connects an engine’s crankshaft to the car’s wheels. The speeds at which the engine and wheels spin are directly proportional, and are measured as a ratio of how many crankshaft revolutions produce a single wheel revolution.

As the car accelerates, both wheel and engine speed increase together. Eventually the engine reaches the upper limit of its speed range. If the car is to move faster without damaging the engine, then the ratio of engine revolutions to wheel revolutions must decrease.

The transmission does this by changing to a different gear. The higher the gear, the slower the engine turns as compared to the wheels.

Imagine you are trying to ride a bike to the top of a hill. Starting at the base, if you were to try to go straight up, you would not be able to move. So, instead, you point your bike mostly “around” the hill, with just a small angle to start climbing the hill. Once you get some speed, you can turn to a larger angle to climb the hill more quickly. It is “harder” to climb the hill at a steeper angle, but it is easier once you have some momentum.

For a car, that “hill” is just the total speed, and the “angle up the hill” is the gear ratio. In 1st gear, you are “climbing the hill”, or speeding up, very quickly; but, you are able to start moving. Once you get moving, you shift to 2nd gear. That is like increasing your angle up the hill, but in a car, it is just the next range of speed. Once you are at a “good” speed in 2nd gear, it means you have enough momentum to shift up to 3rd. Note that if you had to slow down (in a car, the speed limit might decrease; in the bike metaphor, maybe there is some gravel you have to avoid), you have to decrease your angle/shift to a lower-numbered gear. If you do not, you will continue to slow down until you fall off the bike or you stall the car’s engine.

This metaphor is not perfect, but hopefully, it helps get you pointed in the right direction.

If I misunderstood your question, and you were asking something more along the lines of “how does a car’s transmission shift” or “why do you shift gears in a car”, my apologies.

Are you talking about the actual, toothed-wheels inside the transmission? The best approach I’ve seen to answer that question is an old (1936) video titled [Spinning Levers](https://www.youtube.com/watch?v=JOLtS4VUcvQ). This video starts from the concept of a “lever” and builds it up into a simple, yet functional, manual transmission.

Or, are you talking about the “gears” controlled by the lever marked “PRNDL” in your car? To understand that, you’ll need to understand the need for transmissions in general, and we can move on to automatic transmissions in particular.

I feel like I can answer this because I know as much about cars as a standard 5 year old, but I can drive stick!

Basically the lower the gear, the stronger the car is, but the higher the gear, the faster it can go. So if you need more power (like to start moving the car if it’s stopped, or if you’re going up a steep hill) you need to use a low gear. But as you’re driving, you shift to higher gears to speed the car up.

A car’s engine only has a particular range of speeds that it can properly run at. Too slow and it’ll stall. Too fast and it’ll blow itself apart.

The gears on a car follow the same principle as the gears on a bicycle, though they’re more complicated. The principle is to use bigger or smaller gears so that no matter what speed you’re making the car move, its engine is able to stay within its happy little range.