Counter their offer with the math reversed. Instead of subtracting $13k pa for the use of a company car for personal use on the weekend, 2 days out of 7, propose that you use your own car 7 days out of 7, 5 of which are work days. Since they've established that a car is worth $6500 per day over the course of a year, (5 work days) x ($6500), add $32,500 to your proposed yearly salary.
What are they gonna say? $6500 per day is too much? It's their number.