>For example, if you believe I'm making poor use of my car and you think you could do more good with it, does that make it OK to steal it
I think that one should be skeptical of one's own rationalizations for something like that, and also consider all of the costs to everyone. Even if a law is unjust, the machinery that enforces it is going to use resources when the law is flouted. Even if there was no enforcement, any precautions against theft are going to have a cost. Lack of trust has profound effects indirectly.
How do you decide when to apply this kind of thinking?
For example, if you believe I'm making poor use of my car and you think you could do more good with it, does that make it OK to steal it, on the basis that humanity still has the same number of cars and now one of them is being used more effectively?
I'm not saying it's wrong to pick a dollar up off the floor, just that I'm not convinced the reason it is OK is the reason you gave. I think the reason it is OK to pick a dollar up off the floor is because that dollar basically belongs to nobody at that point.
Casey Neistat has a good video on this topic: The Ethics of Stealing a Bike ( https://www.youtube.com/watch?v=wW6gGyfxn1U ) where he argues that it's never OK to steal someone's bike, but it is OK to steal "nobody's bike", i.e. a bike that has clearly been abandoned.