Another way to do this is to get a Bugatti Veyron 16.4 which is available for purchase in the Used Car Dealership or the Online Collector's Dealership for 2.000.000 Cr. (need to be Lv. 21). You can also win it in the GT World Championship Series (Professional).
When you have it you'll need to buy a customizable transmission (20.000 Cr.). Now choose a wider gear ratio in the car's settings. To make it easier you can simply choose what speed you wanna reach and the ratios will adjust accordingly. Then just drive Circuit de la Sarthe without chicanes and the trophy is yours.
UPDATE: As of today, 31st May 2014, the online part of Gran Turismo 5 is no longer available. This includes the Online Collector's Dealership so the only place to get the Veyron is the Used Car Dealer.