Thats very dependent on the car. In my old car it was sometimes an issue on rough Australian country roads, so I had to speak slower and louder. The new car is not a problem as the road noise is much reduced, and I also believe the single hands free microphone in the old car has been replaced by a dual microphone with noise reduction in the new car.
Using the phone itself, I rarely ever had issues with Siri not understanding me, and the only times it failed were usually network issues.
The question then is does the iPhone 6 have such a processor that would allow it to use Siri hands-free without the phone needing to be plugged in at that time. So far it doesn't seem like there's any information pointing to that, so that likely means it will work the same way it will work on iPhone 5s and other earlier phones that support iOS 8.