There are several things that you can do.
First, the easiest thing to do is to have all employees log how they use the car. In other words, have them sign in an out when it comes to car usage. This will hinder them from using it for personal usage.
Second, if this is not plausible, then you can calculate the amount of miles that a person needs to drive a week or month and have the company only pay for that amount of gas and toll. This will also hinder people from using the company car for their own purpose. And even if it does not, it is fine, because they will have to pay out of their own pocket the excess mileage and tolls.
Finally, you can take car privileges away and have people take public transportation. If this is possible, it will save the company a lot of money. A monthly bus or train pass cannot really be abused and you save money on car maintenance.