Measure the drag coefficient of your car

Step 2 Background Information

First, let's define some quantities:

   Fd is the force on the vehicle due to air resistance (drag) in Newtons
   Frr is the force on the vehicle due to rolling resistance in Newtons
   F is the total force on the vehicle in Newtons
   V is the vehicle's velocity in m/s
   a is the vehicle's acceleration in m/s^2
   A is vehicle frontal area in m^2
   M is vehicle mass including occupants in kg
   rho is the density of air which is 1.22 kg/m^3 at sea level
   g is the gravitational acceleration constant which is 9.81 m/s^2
   Cd is the vehicle's drag coefficient we want to determine
   Crr is the vehicle's coefficient of rolling resistance we want to determine

Now for some formulas:

   Fd = -Cd*A*0.5*rho*V^2 (formula for force due to air resistance or drag)
   Frr = -Crr*M*g (formula for force due to rolling resistance)
   F = Fd + Frr (total force is the sum of Fd and Frr)
   F = M*a (Newton's second law)

Note that both Fd and Frr are negative indicating that these forces act opposite to the direction of the velocity. Note also that Fd is increases as the square of velocity. This is why driving at high speeds is much less efficient than driving at low speeds. Combining these formulas with a bit of algebra gives us the acceleration due to air and wind resistance as a function of velocity:

   a = -(Cd*A*0.5*rho*V^2)/M - Crr*g

Note that the acceleration is negative indicating that air and wind resistance will cause the velocity to decrease.

I created a spreadsheet based on these formulas to generate a model of velocity vs time that can be compared to actual data. The model values for Cd and Crr can thus be adjusted until the model matches the data. This adjustment can be done manually, by overwriting the values of Cd and Crr with new values till the model matches the data, or it can be done using a "Solver" function.