No faster than other flies, the 100 MPH doesn't figure into the equation, the fly speed is relative to it's surrounding inside the car.
And that is the real question. Relative to what.iyou could consider the rotation of the earth at 1000 something.mph. how about the motion around the sun. Or we could just follow today's thought and keep it simple and say bout 10 mph.
If you are driving down the road in your car at 100 mph, and a fly takes off from the back seat, and lands on your dash, how fast was he flying?
If he can make it to the dashboard, that's one mighty fly - it would have to break the sound barrier to do so; but the minute I noticed it, the windows would be opened and it would get sucked out of the car !