In countries with hot climatea the air conditioners of cars are mostly used for cooling the temperature inside the cars. This adds to comfort but at a cost. The mileage is reduced which directly adds cost to the owner of the car.
The roof of the car can be modified as shown. The upper portion of the roof is made up of insulated material or a composite structure, which has a low thermal conductivity. This prevents the sunlight/heat from entering into the car. The lower portion is "fins" made of high thermal conductivity material. When the car is moving the fins help in expelling the heat from the car because of forced convection.
This arrangement will help considerably in reducing the temperature inside the car and prove to be cost effective.
The concept sketch of the above is shown in the PDF ‘ROOF’.