The SLAC ATLAS group has a CERN car, license GE 606074. You need a CERN Driving License (go to EDH, and click Access Request) to use it, and a Mission Order for offsite use such as going to the apartments.
As of March 2010, there is a new way for filling up. Go to the fuel pumps on the west side of Building 130. There is a badge attached to the car key. Hold it next to the icon on the pump. It should display the CERN Car number (3183), and then ask what fuel you want. Choose unleaded using the corresponding number code. Enter the mileage. Now you can fill up. This facility is supposed to be operational 24/7.