Otherwise, the faster the car.
The greater the air resistance.
The engine is easily damaged.
There is also a physical point of view. If the car glass is straight, people in the car will mistakenly produce a virtual image in front of the driver because of the imaging of the flat mirror, thus causing interference.
If tilted
It's like it's up there
do not affect
But there will be no upward phenomenon when the bus is high from the ground.