Apologies if this is somewhere else, but this way is fast and very accurate.
1. Find yourself a time somewhere between morning and afternoon (with good light).
2. Measure how high the car is, roughly.
3. Take a picture from directly in front of the car, at a height a half of your car's height, at least about 10m away from your car.
4. Download the Free and Open Source Software program, "The
GIMP". GIMP stands for GNU Image Manipulation Program. If you use Ubuntu, it is in the repositories (synaptic or add programs).
5. Open up the picture in GIMP. Notice the point on the car for maximum height. Go out and measure from the ground to this point with a tape measure, taking care to eliminate parallax error from your measurement. Note this. Then measure the distance from the outside of the right front wheel to the left front wheel. Go back inside and to the GIMP again.
6. You will want to select around the car, so that nothing sticks out beyond your selection. Leave a little border around the car. Use the rectangle select.
7. Go to Edit -> Copy, then Edit -> Paste as New. This will have the car zoomed appropriately, if not, use the zoom commands in the View menu.
8. Save the new image.
9. Press "b" for "Create and edit paths". Or it's the upper right icon in the toolbox.
10. Select around the outline of the car as best you can, for curves, you will need more points than straighter sections. Try and be as accurate as possible. When you are done with the car and your dots are almost at the beginning, select "Create Selection From Path". You should get a nice little flashing outline of your car.
11. Go to Layer-> New Layer, and create a new layer with fill type transparency.
12. Use the bucket fill to fill the selection.
13. Go to Dialogues, Pixel Count. That is the number of pixels in your selection, which corresponds to the frontal area of your car in pixels. Note this figure. Now you just have to convert pixels to real area (m^2 or feet^2).
14. Measure the height in pixels of the highest point of the car, to the ground, using the measure tool (looks like a drawing compass in the toolbox). Or Tools -> Measure.
15. Measure the width in pixels between the outsides of the two front wheels (same span measured in 5.)
15. Once you have this, get height/pixel = Height obtained in step 5/ Pixels measured in Step 14. Should be a small number, much less than 1.
16. Get width/pixel = width obtained in step 5/ Pixels measured in Step 15.
17. Multiply the two number obtained in step 15 and 16. Should be a smaller number yet, even closer to zero. This gives you area/pixels.
17. Multiply the number obtained in 17 by the pixel count obtained in 13. This will give you the frontal area of your car in whatever units you were using.
Now that you have a very, very accurate frontal area of the car, you can compute your drag coefficient more accurately, once you have CdA (which the coast down test will give you).