Here in the USA, where Regular Gas is $4.00+/gallon (with Premium Gas is $4.50+/gallon) - this could be helpful. :-)
I'd heard about these 'billboard' cars before, but not with free gas.
See http://www.cnn.com/2008/LIVING/06/19/gas.incentives/index.html from CNN.com.
In the other incarnations of the 'billboard' car, you (driving a suitable vehicle ) get a wrap created for your car (with the ad on it, of course), which may or may not change anytime during the length of your contract with the agency, usually for a flat monthly fee. I hadn't seen it where you work for the company doing the advertising, and supply your own car.
(I used to get a car allowance of $250 per month for the consulting company I worked for as a full-time employee. I could have let them supply a Ford Taurus for me instead. No advertising on either car).