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

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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?

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Anonymous 0 Comments

As pointed out this is engine breaking and it will apply to cars. If you have been properly trained how to drive you should know about this, but really few people are.

Case in point,
I was coming down a canyon in Utah from my favorite resort. It is pretty common to smell brakes from people not knowing how to do this. On my last trip we are about 2/3 of the way down and we come upon a mid size SUV with smoke coming out from the front wheel wells and two very concerned 20 somethings.

I stopped and explained they should actually keep going but keep the car in Low 2 (an automatic), and this will keep the car below 30 or 40 mph. They seemed to understand.

We continued in ahead and stopped for our usual snack, maybe 15 20 minutes, and then continued back to our hotel. 5 min late we are stuck in a line of 20 cars, doing 35 mph.

The same two guys did not realize ( ok I did not explain it all) that once they can drive normally they can shift to D, normal mode… they had just kept driving in L2 for miles.

I felt bad for that rental car….

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