Auto Racing: Aerodynamic Drag improvement in cars, drag coefficients, aerodynamic drag

Question
Hi, I am a mechanical engineering student and I am doing a project requiring me to do research into reducing the aerodynamic drag coefficient of a car. I have done some testing already on a few cars for the purpose of calculating their aerodynamic drag coefficients. However, I am confused about some results that I am getting. SO far I have noticed that my aerodynamic drag coefficient is changing for the same car at different velocities(increasing with increased velocity). Is this supposed to happen? I am doing my tests within the range of 50 - 20 km/h. Also when calculating the aerodynamic drag coefficient, am I supposed to use the change in velocity lets say, every 5 km/h drop, or am I supposed to use the actual velocity at the corresponding drag force? Any further assistance related to my project would be greatly appreciated. THANK YOU SIR.

Answer
The drag coefficient (cd) of a vehicle should not change with velocity since cd is a normalized value. What is happening (more than likely) is that the vehicle's attitude (pitch) may be changing as you increase speed due to the compression or change in your suspension system. Also, there are many more inputs that would have to be taken into account depending on how you are measureing drag.

In a wind tunnel, we would measure the axial force of a vehicle with a balance of some sort. The output of that device would be in...pounds. We would also measure the dynamic pressure at the vehicle in...pounds per square foot. Therefore if we know the frontal area of the vehicle, we can calculate drag. If we only have velocity, we would calicualte dynamic pressure (q) with the follosing equation:

q = 1/2Rho*velocity^2

where Rho is density, of course all of your units have to be appropriate.