system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why did some older semi trucks have two gearboxes? And what's the point of some of then having so many gears?
gpt
Semi trucks only have 10-15 horsepower per ton of weight that they're moving, whereas cars have several times as much. And the truck's engine only makes that much power in a narrow RPM window, say from 1,600 to 1,900 rpm

You need a lot of gears to keep the RPM jumps between the gears as small as possible so the engine can always operate inside its RPM sweet spot. That wouldn't work if they only had say 5 or 6 gears. So that's why they have so many.

Now, you can either design a fairly complex gearbox with like 12 or 14 forward gears, or you can combine two of them. Basically you have a 6-speed gearbox and another one that allows you to 'sub-divide' those 6 gears.

Instead of just having 4th and 5th gear, the secondary gearbox lets you select intermediate steps.