I had a flight the other night with a 220 knot tailwind which cut an hour off our time enroute. I made the usual anouncment telling them the weather at the destination and also said we would be arriving an hour early due to the strong tailwind which was giving us a groundspeed of around 730 mph. After everyone got off the plane one of the FA's said some passenger told her that I was wrong and there was absolutely no way we were going that fast because if we were we would have been breaking the sound barrier and he knew that was not possible in our airplane. I just wish he would of told the captain or myself that so we could explain to him the difference between groundspeed and airspeed.