Please ignore this if you are just using a car battery as convenient portable power for some gadget in a field somewhere!
However if your gadget is going to be fitted to or used in a vehicle, I'd be very wary of transient ground potential differences in an automotive environment. If the transistor emitter, Arduino Ground and buck regulator ground aren't all connected to the same point by relatively short wires, you are likely to have trouble.
Also, is the buck regulator rated for automotive use? The transients it may be subjected to are absolutely vicious, and even on a modern well maintained vehicle are not nice and well outside the abs. max. ratings of most commodity switching and linear regulators. Google: "ISO 16750-2" or the superseded standard "ISO 7637" which may be easier to find the full text of as ISO charge for access to their standards.