Why do semi trucks have so many gears?

The BIG diesel engines that are used in semi trucks have a very narrow power band, and the driver needs to shift frequently to keep the engine running at its most efficient speed. Sometimes there's less than 750 RPM between gears.

Semi Truck Gears

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This question is impossible to answer because of many factors 1. the gear ratio of the rear end determines how much speed or pulling power is created by the engine and transmission. The greater the speed, the less the pulling power 2 Trucks have a huge variety of transmissions... All of which shift at different speeds and rpms A good example is the truck i drive. It has an 18 speed transmission with a 25 percent underdrive auxiliary. This means that the truck has 36 forward gears. Ranging anywhere from 1 mph max speed, to 75 mph max speed. most trucks have 9 speed transmissions, which have a greater spread between the gears. it means the rpms create greater speed between shifting. hope this helps

Like cars, they can have many different engines. When I was in the military they always had the "lowest bid" trucks with the smallest engines. Without a trailer attached a small engine "yard tractor" might only be able to go about 65-70. Something like a big engine freightliner with no trailer maybe 110, give or take in the highest gear. Most highway tractors are geared so low in first that if you bring it up to the red line and let off the accelerator it will drag the tires. Have to have that to start off with a heavy trailer uphill. All the other gears are spaced out in between until you hit the final gear and overdrive. With no trailer or an empty trailer you would start out in about 3rd gear and go up through the gears from there.

Because of the heavy weight they are capable of moving. Transmissions primarily multiply torque. A fully loaded 18 wheeler requires much more torque to get it moving from a dead stop than it does to keep it moving at highway speeds.

A typical American Highway tractor with a 13 speed transmission pulling an empty trailer can easily start off in 5th gear on level ground without doing any damage to the clutch, engine or transmission. When you load the trailer to its maximum, you are going to need to start off in a lower gear.

I've driven tractors with 7 speed transmissions, 8, 9 (and 9 speed "semi automatics"), 10, 13 & 15 speeds (there are even 5 speed automatics made for heavy trucks). With all of those, I rarely used 1st gear to start off in except when I was starting on a hill or the truck was loaded all the way to 80,000 pounds. Very often you can skip gears as well, like going from 5th to 7th, for example, with no ill effects. Even though the truck may have a 13 speed transmission, an experienced driver won't necessarily go up through every one of them every time, from a dead stop.

18th gear at 50 mph. Lots of shifting required to get there. But even at 105,000 lbs I still start in 3rd. When you have a 2 speed differential shifting from 10th to 11th will get you from 1700 RPM to 1500 RPM. The more gears you have, the easier it is to stay in that 1500-1800 RPM "sweet spot" for maximum power.

It is because of the limited RPM range of the engine, most only go to 1950.