system
Given the following post, respond as if you're commenting on Reddit:
human
[Mad Max] how is knowledge of mechanical engineering preserved?

So a lot of factions have vehicles - bikes, cars, and most of them are cobbled together from lots of different parts. But how do they know how to make working, driving cars? How do they know how to keep an engine running, or how to fix issues?

The only three possibilities I can think of would be these:

1. It's a broadly master-apprentice system, where someone who has the knowledge takes an apprentice and teaches them everything they know. But is this realistic in smaller groups?

2. They learn from pre-collapse material, like engineering textbooks and manuals. But these are usually made with the expectation of a full school education in mind, so how useful would they be? Also, you'd need to be able to read.

3. They reverse-engineer existing vehicles. So your warband manages to tear the tires off some unfortunate wastelander's car, you murk them and then take the thing back to your hideout. But there the problem is that it's an endless cycle.
gpt
Mostly 1, there are crews that preserve that knowledge and they pass it onto the younger generation.  Its most realistic in the smaller groups, but the knowledge is very specialized.  They don't know how to fix cars, they know how to fix THAT car.  The knowledge itself become nearly religious in nature.  Given that the vehicles are typically critical to the group's survival, preservation of that knowledge is a survival activity.  

Two is also a possibility, but most humans do not know how to read.  Some still do, and those kinds of humans will probably have some manuals about.  Further, in some of the manuals there are enough pictures that some processes can be replicated without a firm grasp of the text.

Three is more scavenging than reverse engineering.  You have two broken cars, the more broken is stripped and now you have one working car.  The cars that they use are also very different than the cars we do.  They are much more similar to cars from the 50's, where there are no significant electronic components.