In addition to the answer posted by Jack, there is also buoyancy that must be accounted for.
By shear force, six inches of water will sweep you off your feet. Effectively what could happen is the same as taking a dive into a shallow water slide- possible head injury and the possibility of drowning, alongside being swept away. Those same six inches may flood your car.
One foot of water, even stagnant water, will float your car. If the water is rapidly moving, I'd hypothesize it may even roll your car, maybe even carry it off.
An EF2 tornado has wind speeds of 111-135 mph and can lift cars off the ground, but might not be thrown. An EF3 tornado, with wind speeds between 136-165 mph, will throw cars.
Very few people are foolish enough to drive into tornadoes (the tornado interceptors are an exception- they have proper equipment). The same can't be said about floodwaters. I've seen people drive through unsafe waters during a rain storm and have their cars stall in the middle of the road.