The EPA revealed in a recent survey the Veyron uses 29.4 litres of fuel per 100km during city driving and 15.5 litres/100km on the highway, making it the most frowned-upon current production car the EPA has seen. The quad turbo W16 was not really developed with fuel efficiency in mind though.
If we hypothetically halve the engine, so it's a twin turbo V8, then it chews just under 15L/100km in the city and around 7.5L/100km on the highway. That's not too bad for what would be a 373kW V8. Fortunately, it's not simply a V8, it's a 746kW, 8.0-litre W16 capable of pushing the exotic Frenchy to 407km/h.
The best, most green car on the market then? The Toyota Prius of course. The Americans love the Prius, especially celebrities wanting some 'see, I'm intelligent' attention. It was credited for it's fuel consumption rating of 4.6L/100km.