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I'm trying to understand why cars have transmissions – why is it necessary to change from gear to gear as you're driving. I've heard the rule of thumb, "the higher your speed the higher your gear, the lower your speed the lower your gear," and I'd like to know the reason why it works this way.

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    This question appears to be off-topic because it is about maintenance or repair – Gabriel Mongeon Jul 23 '13 at 14:36
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    @Gabriel, I tend to disagree. It is about how the car functions; it is not about driving techniques. It isn't asking how to drive; it's asking why the car is designed the way it is. Without knowing why a car functions in a particular way, repair is impossible. – anonymous2 Dec 22 '16 at 23:29
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Get on a mountain bike and change between gears. The same reasons apply to changing your car's gears and changing your bike's gears: you can't pedal faster than x, and you can't get away when the gears are too extreme.

The point is that humans and engines operate best at a certain cadence or RPM. If the engine spins too slowly, it can't put enough power down. If it spins too fast, components will endure too much stress, overheat and break quickly. A formula 1 car can rev up to 18000 RPM, but the engine needs to be rebuilt after every race. A large truck only revs to about 2000 to 3000 RPM, and while it's pretty strong, it can't go very fast.

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Conventional – non-electric – cars are powered by internal combustion engines, those engines have to be running (spinning) to develop power and develop their best power over a relatively narrow RPM band – maybe 1,500 to 2,000 RPM. When you shift gears what you're doing is to matching the engine speed to the speed you want to be traveling down the road. This lets you operate the engine at a relatively fixed "good" speed while allowing you to vary your speed.

The relevance to maintenance and repair is that frequent operation outside of that band could damage the engine. If you run the engine too slow, called "lugging," the pressure in the cylinders can become high enough to do damage. If you run the engine too fast, "over-revving" it, you run into another set of problems. At high speeds you will see more heat build-up and problems like "valve float" will start to creep in. Since many parts of the engine are spinning and changing direction the balance of parts like pistons, connecting rods, and the crankshaft all become critical as the engine speed goes up. The transmission and shifting gears gives you a way to avoid these problems.

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Gears are required to keep the engine at rotational speeds where it creates a desirable amount of power across a wide range of road speeds. Generally, the faster an engine turns, the more fuel it consumes, and the more power it makes. Engines are also only capable of rotating to a maximum speed (redline).

If a vehicle was limited to a single gear ratio:

  • Power at low speed would be very bad, as the engine would also be turning very slow.
  • Fuel consumption and wear at high speed would be very bad, as the engine would need to be turning incredibly fast.

Gears are designed for a given limited range of road speed. First gear allows the engine to provide a large amount of power at very low speed. Once a reasonable road speed is achieved and the engine reaches the upper portion of it's usable range, a higher gear is selected. This positions the engine speed down toward the lower end of it's range for the same given road speed. The process is repeated until the vehicle reaches the point where the gearing provides the highest possible vehicle speed matched with the highest possible engine power. An overdrive is usually fitted, which gears above the highest power/speed combination, for the benefit of reduced fuel consumption.

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