A car is travelling in one direction at 80 km/h and a fly is coming from the opposite direction at 5km/h. (So its velocity is -5 km/h, since it's in the opposite direction.)

The fly hits the windshield of the car and is now travelling at 80km/h. In order for the fly to achieve the speed of 80km/h it's speed must have passed from -5km/h to 80km/h. (Meaning it must pass through the speed of zero) Therefore if the fly passes through a speed of 0km/h and the car is in contact with the fly the car must also pass through that speed of 0km/h.

This seems to mean that everytime a car gets hit by a fly it will completely stop. Why is this not so?