This offer began at 12:01am PDT (8:01am BST) today, and will end at 11:59pm PDT (6:59am BST) on Sunday 7th September. The entire game is available for players to get stuck into, allowing them to also compete in a special endurance event this weekend - the Grand Tour of Prague - which will earn a new title and badge for their Forza 5 profile if they drive a minimum of 130 miles in the race.
Any achievements and additions to Gamerscores will be kept if you decide to buy the game after the free time with it ends too.
For Xbox Live members in the US there's the chance to qualify to win a paid trip to the three-day Forza Motorsport Grand Prix Road Racing Class, which will take place at the Bondurant School of High Performance Driving in Arizona. A video giving more details on the prize is below.
The next title in the Forza series, Forza Horizon 2, is due for release on 30th September for Xbox 360 and Xbox One.