The online tool that is supposed to tell you if your phone will work with Android Auto is decidedly not working.
Unfortunately, it's broken. It's reporting back that the Nexus 6, Moto X 2014, HTC One M9 and HTC One M8 are not compatible with Android Auto. But we've used every single one of those phones with Android Auto. (And we're testing more as time permits.) So for now, don't rely on that check page. We'll update when it's fixed.