Not actually
that dumb of an idea. It actually works if you happen to have a nice, windy dry lake to play on:
https://en.wikipedia.org/wiki/Blackbird_(land_yacht)
But for an ordinary vehicle, I think the biggest drawbacks are 1) carrying around all that excess weight when the wind's not blowing; and 2) chopping up other vehicles, pedestrians, and so on with the propellor :-)