Your forgetting the fact that most stones that are thrown by the wheels are actually getting caught in the tyre grooves and then let loose when the centrifical force overcomes the friction the tyre grabbing them is creating.
I actually think that most stones will actually fly off the wheel at a much higher height than what you are calculating.
The reason why I think this is when you run slicks and they are cold the stones that are thrown up are alot less (observed by less stone noise under the car) But when you come in on hot slicks (they are like glue for gravel and rocks) the noise starts up again.
Also when I used to run wets or grooved tyres the noise is would be far more noticeable again..
This is a race car so it probably a little different from the real world.
I have to run over gravel and dirty car park before getting onto the track.
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