Here's another method:
1) position the car parallel and close to a wall
2) make a second cardboard "wall", 90 degrees to it, at least half the width of the car
3) roll the car back and forth through the cardboard wall, trimming as you go
4) photograph the results
(edit - no telephoto lens required, since you're taking a pic of a flat surface) & do the pixel counting thing
eg:
Hey, I'm not saying it's practical/easy - or more fun than punching the car through a snowbank. Just an idea.