Before I start modelling a car I always start with gathering informations about a car. Try to find best possible photos with highest resolution. When I have that then I start with photo matching in Image modeler software. Usually 5-6 images will do the job and after creating reference points all across the images I can get accurate cameras which I can import in 3ds and then create spline cage. Here you can see early spline cage and camera with backplate in 3ds max:

After I have nice spline cage the modelling begins. Using all cameras setted up I try to make simple base of the car. When I'm sure about how the base is looking I can collapse my base with turbo smooth modifier and then I can start with detailing parts of car and cutting everything.

Volvo S80 was created from photos from I think "Geneva International Motor Show". Here it is finished.

This is old project from 2013-2014 however making simple base model is the best way for me for creating high end models. Not only automotive but also anyother things like furniture etc.