I am driving a car provided by my employer, and it is understood that I am using it for personal use as well. I have a company credit card that I use for gas. How much (if any) gas am I expected to pay for out of my own pocket, assuming I drive a fairly reasonable amount outside of work?
In the United States there is freedom of contract (assuming you are working for a private employer), and you are free to make your own deal. Remember that the government probably considers it to be “income” if you get a car for personal use, and will want to find a way to tax you on it.
The norm is for you to pay for your own personal gas use. Keep track of how many personal miles you log, how many corporate miles you log, and split the gas costs accordingly.