Calculate Drag Coefficient

Background: simulate the drag coefficient for a truck model

I have read the following post
www.comsol.de/blogs/how-do-i-compute-lift-and-drag/
and looked at the flow over an Ahmed body example
which shows that the total drag force in the Y direction is: spf.T_stressy

To calculate c_d i need to calculate
c_d = \frac{ 2F_d}{ \rho u^2 A} (en.wikipedia.org/wiki/Drag_coefficient)

where (please correct if wrong):
F_d = spf.T_stressy
/rho = spf.rho
u = spf.u

Is the above correct? How do I calculate Area? The area should only be the surface exposed in the Y direction (spf.nymesh?)?

thanks in advance!
Jess