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Bugatti Veyron - fastest production car ever - heck, faster than Formula One cars. :)

407 km/h (255mph), proven on the TV Show 'Top Gear' on Volkswagen test track in Germany.

its not FIA restrictions, its track restrictions that limit their top speed. the wings create massive downforce which helps them stick to road while cornering, but on straight aways, especially past 200mph, the downforce just keeps increasing which also makes the car in effect 'heavier' as its pushes to the ground, so unless they radically redesigned their wings (and become almost undriveable even with slightest wheel turn), they would have issues hitting 255mph.

the Veyron has hydraulic adjustable wings to switch between low, mid and high speeds.