The last few times I've rented a car, I have taken a pix of the car and a closeup of the license plate with my smartphone before I drive off the lot. Serves two purposes.
1. I email the photos to my spouse so he has them for reference in an emergency.
2. When I check in to my hotel, I have a handy reference of the plate number for the hotel registration form. No more running back out to the parking lot to jot down the number. Just open the phone and check the photo.