Mechanical Engineering

It varies.

You WANT to be as loose as possible while still sserving the function of what the part is supposed to do. The reason for that is every time you tighten tolerances, prices skyrocket to make that part. Drill a hole is dirt cheap, but you will get maybe +/- .050" (roughly +/- 1mm). Milling a hole is more expensive but you can easily hold +/-.005" (roughly +/- 0.1 mm). Then you can ream a hole and hold it to +/-.0005" (roughly +/- 0.01mm). Each different operation makes that feature roughly an order an magnitude more expensive. Not the whole part, mind you, but if you broken down what each step in the manufacturing of a part cost, each step would get more expensive by roughly 10x.

In general (and I am speaking in very vague terms because there is so much variety out there), but holding a machined part to +/-.005" is fairly typical for the more important dimensions and the less important ones are +/-.015" or even looser. You would only ever go to super tight tolerances when you absolutely need them - like if you were press-fitting a bearing onto a hole. You need that hole (bore) to be a very tight dimension. Thats when you would be asking for the kind of tolerances that Musk is asking for ALL PARTS. His quote - if it isn't fake - shows such a complete and utter ignorance to manufacturing it is painful to real.

A plastic part in your car is probably made to roughly +/-.010". A steel or aluminum machined part is probably made to +/-.005". Sheetmetal would be probably +/-.015" Some dimensions within that part might be looser, and some might be tighter if they were mating with another part. Musk is asking for all dimensions on all parts be +/-.0004"