They can. (But that’s not always the reason for more gears.)
Gas engines run most efficiently at particular speeds. Too fast, and the gas in the combustion chamber isn’t completely burnt before the cylinder is opened. Too slow, and there’s little power, because cylinders are firing slowly, and not much energy is being generated.
So the idea is to keep engine revolutions close to an optimal range. When a car is going slow, the wheels are barely turning, and the ratio of RPM to wheel spin, which is handled by the gears, is high. As the car speeds up, a different gear ratio becomes more ideal.
If there’s a wide selection of gears, then the car spends less time outside the engine’s optimal economy range.
For driving at constant full speed, the addition of overdrive gears is useful to save gas. Those are gears that reduce a car’s power and acceleration when full engine power is not needed.
Diesel trucks have many gears for a different reason, which is that diesel engines operate in a more limited range of RPM.
But another reason for more gears is to stay as close as possible to a gas engine’s maximum RPM — where it’s delivering the most power. This is typical of sports cars. In sports cars, more gears are used to stay as close as possible to the red line — to consume as much gas as possible!
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