There can be only two possibilities:<br>1. In case the plane and the car were moving in the same direction the plane was indeed flying with slower speed than the car.<br>2. In case the plane was flying with greater speed than the car the plane was going in some other direction. The velocity of the plane in the direction of motion of the car matters.<br>For example, if a helicopter goes up at an angle of $45^{\circ}$ with a speed $150km/hr$ and a car right below also goes with the same speed, the car would see that the helicopter is going with $(150-\frac{150}{\sqrt2})km/hr$.