Why do trucks have so many transmission gears?
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Diesel engines have a much narrower power band than gasoline engines. The power band is the RPM range between peak torque and peak power.
The power band on a gasoline engine found in a consumer sedan might be from 1,800 RPM to 6,000 RPM. By comparison, the power band on a semi truck may be 1,400 to 2,200 RPM.
Operation outside of the power band is inefficient, energy from combustion is wasted by heating the engine block or exhaust rather than being transferred to the wheels as motive force.
The high number of gears in freight hauling trucks is necessitated by the need to accelerate a very heavy load from a complete stop, and by the need to keep engine RPM low while at expressway speeds so as to not waste fuel.
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