Rather than taking measurements, tape a 16x24 framing square onto a reasonably flat surface of the vehicle then take a picture trying to keep the camera lens parallel to the vehicle. As in minimal parallax. Import the photo into whatever software you're using, draw a rectangle around the framing square, them map the photo into that rectangle. Resize the rectangle with the photo mapped into it to 16"x24". Unmap the photo. Now you have a full size picture of the vehicle. Layout whatever it is you're laying out on this picture. You won't be off by much. Not enough to worry about.
Much better than taking a bunch of measurements and usually more accurate.
Much like Bob said, I use a 1foot square bit of magnetic sheet and slap it on the door, stand back 25-30ft and zoom in to take picture, this take out some of the curve you get from being to close. Import picture and resize until the square I put on the car is 1ft x 1ft ........ start designing.
You can also check if it's right by the wheel rim, measure the rim i.e 16.5" and resize pic until the rim is the right size.(make a wireframe 16.5" circle and place over top to check) Practice taking pictures with your own car and working on it in AI.
Also, when resizing the picture, do it proportionately or the picture will be wrong, ie if you drag it out horizontally, it needs to go vertically the same percentage.