On the basis of resistance provided by the objects to the air, objects can be divided into two parts.
First one is aerodynamic and second one is non-aerodynamic.
An aerodynamic object offers least resistance to the air while either the object or the air is moving. On the other hand the resistance provided by the non-aerodynamic object to the air is much more than that of aerodynamic objects.
Here we are interested in aerodynamics in context of automobiles.
Aerodynamic and cars
As we have discussed above that a non-aerodynamic object offers more resistance to the air which comes in its path, let us understand how this affects our vehicle performance.
In a simple experiment scientists have proved that a bicycle can achieve more speed which is running behind a truck or bus, rather than the one which is facing direct wind. In first case when bicycle runs behind a bus or truck than the air thrust is mostly taken by the front vehicle and the Bicycle has to take very less thrust, hence a bicycle rider can achieve more speed for the same power.
The resistance that air offers to a body is non-aerodynamic, adds to the load of the vehicle and requires more power to overcome it. To acquire this extra power an automobile has to burn more fuel. That is why, Modern day cars are more streamlined than what they are half a century ago.
In case of racing cars, one cannot bear the air resistance, since it reduces the vehicle speed for a given power engine. Hence highly efficient aerodynamic cars are used in car racing. By virtue of their aerodynamic shapes these modern-day racing cars are so efficient that while running inside a cylindrical tunnel they can overcome their weight and run on the roof of the tunnel.