It’s only always on if you leave it intact and functional. If you build the always-on car for the purpose of one specific trip - perhaps to deliver life-saving medicine to a city where thousands of lives are in danger if they don’t get the medicine in a time frame that requires the car to get it there - and then disable/destroy it, it’s no different in terms of pollution than driving a regular car for it.
I’m sure you can see the analogy, but just to be complete: if you animate some undead for a particular good task and then destroy them, it’s analogous. And yet it’s evil to do so, no matter how good your purpose, nor how brief the polluting undead exist.
Which is another reason the pollution thing is silly: it depends on how long the undead last! As opposed to being an evil act because the act itself is evil.