Aerodynamics of the new Mirage (0.27-0.31 drag coefficient varies with options)

[MetroMPG]

This technical drawing was provided in the Mirage brochure (PDF) from Australia:



It's a decent candidate image to use because it doesn't have perspective distortion we'd typically get in a front-on photograph. Also, Mitsu conveniently provided exact dimensions in millimeters, so we have an accurate reference to figure out the pixels:mm ratio in the image.

1. First, I enlarged the image to make it easier to work with.
2. Next, I traced the car's outline with the selection tool, then adjusted the selection so it was centered on the line.
3. Then I bucket-filled the selected area with black, and deleted everything outside the selected area
4. I converted the image to black and white (2 colours only) and was left with:
   
   
5. Photoshop has a handy Histogram tool (Image > Histogram) which will tell you the number of pixels for each individual colour in the image
6. Since it's a 2-colour image only, we can easily see that the black pixels represent only the car's frontal area (153501 pixels, in the image I was working with --- a larger one than the attachment image in this post).
7. From here on, it's just math:
   (A) the diagram dimensions showed the width of the car as 1665 mm.
   (B) Using a measurement tool, I found the pixel width of the car in my working image was 457 px.
   (C) So we now have a ratio of 1665 mm/457 px, or 3.643 mm/px.
   (D) But a pixel is a square, so we have to sqare the number of mm to get 13.274 square mm/px.
   (E) Last step: how many pixels in the car? 153747 px * 13.274 mm2/px = 2040811 mm2, or 2.041 square meters.

QED!