I started with the basic car shape, and then made a few variations (taxis, police, …)
After the modeling, I fixed some issues on the mesh, and made the UV of each car, using the same UV map.
The last step was texturing the car. I use the same mesh for each texture variation, so it’s easier using the car variations. For the taxis and police cars, you have to add another model on the basic car mesh, with a special texture on it.