working at a car manufacturer. The MechE probably knows most of what there is to know about the theoretical concepts behind how at least some of the car's systems work. He (or she) also knows intimately about the design of those systems that he has worked on in certain cars. He may or may not know about some other systems even in the same cars. If an engineer worked on the engine of a BMW, he probably doesn't know how most of the chassis computers work, because they are pretty much completely unrelated to the work he did. It is very likely that he also doesn't know much about many cars from other manufacturers, and that he doesn't know a whole lot about how to repair cars, even the ones he has helped design. He may not know how to put them together on an assembly line, either. Don't even ask him to weld something well. And it's likely that there are many other people who know much better how to drive the cars he has designed fast on a race track or back road.