Originally Posted by
guv1976
Of course, if the rental car you pick up already had its own E-Z Pass transponder installed, it will not be possible to register that car on your own E-Z Pass account. You can still mount your transponder on the windshield and use it, but you will not have the protection that comes with registering the car's plate on your E-Z Pass account in the event of a transponder malfunction.
That may or may not be true. I'm not sure that the rental car agencies register each transponder with the specific car it resides in (especially because I think some rental car agencies move them from car to car). But also, even if it is registered, it may be registered in a state that's different than the one I carry in my bag; there is nothing preventing the same car from being registered to different transponders issued by different states. But ultimately this is not a big issue given the de minimus risk of not registering a rental car to the specific transponder being used.