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The larger (and therefore heavier) the vehicle, the more gears are required to accelerate it. Larger engines such as the ones in transport trucks also have much lower RPM limits, so they can not be revved nearly as high. A typical accelerating truck generally revs between 1700 and 2500 RPM. This very short range means far more gears are necessary to permit the truck to accelerate or climb hills.

Without a lot of gears (sometimes as many as 18 or more), the effect would be very similar to trying to start a passenger car from a full stop in 4th gear - i.e: the engine just doesn't have enough power at excessively low RPM.

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16y ago
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Q: Why do semis have so many gears?
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