In order for it to count as a theft, one has to take something from a person and use it yourself in such way that the other person can not use that anymore (unless it is somehow returned it). So, at least two conditions should be satisfied, not just one. In case of gasoline, it is clear cut. In case of joyriding, you are stealing a portion of car's life. The owner can not use that portion on themselves anymore (thus depreciation), but you did.

Scratching car - you did not take anything and used for yourself. Same goes for other things you mention, except possibly the flaying, if you really take that skin for yourself and use it somehow for your benefit. Make a lamp or something. Obviously, flaying would not be just stealing even in this case, because you have induced suffering as well (and possibly death).