Why do trucks have so many transmission gears?
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You can think of torque as the amount of force the engine is outputting. You want a lot of torque for hauling heavy loads (as trucks are meant to do) because it allows you to accelerate more easily.
Being in a lower gear gives you more torque, but less top end speed.
In a regular car you may be able to accelerate a heavy load up to 20 mph in first gear while having enough torque. But then when you switch to second gear your torque will be too low and the engine will struggle to accelerate further.
You can accommodate this by spacing the gears closer together. This means that the amount of torque you lose when shifting up a gear will be less. But in turn that next higher gear will not have as high a top speed.
If you space all the gears closer together your top speed in top gear will be too low. The solution is to add even more gears.
In summary. A car has fewer gears spaced further apart to reach a desired top speed. A truck has more gears spaced closer together to reach the same top speed.
Important bit I left out. Your engine generates more torque when your RPMs are in a particular range. Usually something like 4-6k RPM. When you go up a gear your RPM will drop by an amount proportional to the gear spacing. If you have closer spacing your RPM drops less when shifting up. So with closer spacing you can keep the RPM closer to that optimal RPM where it generates the most torque.
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