Eli5: What is the purpose of downshifting for a semi-truck and how does it work?

Question

I have always wondered what those loud noises are the semi-trucks make as they appear to be slowing down, but I don’t understand how that helps?

In: 547

Answer 1

When you downshift, the RPM on your engine increases without more input of fuel. That is referred to as Mechanical Breaking. Because you use the momentum of the wheels to speed up the engine instead of fuel. It’s safer to, for example, use a combination of mechanical breaking and regular breaking when going down hill.