Before you can boot up and play Forza Motorsport 5, you will need to connect the game to the internet first.
Much like the Xbox One console itself, you will need to connect to the internet to download tracks, cars and other things. From that point onward, you are free to play Forza Motorsport 5 the game offline.
The following was confirmed by Turn 10 Studios boss Dan Greenawalt in an interview with IGN:
“So when you first boot up the game, we’re going to ask you to log in. And when you log in you’re going to get the Drivatars and you’re also going to get a whole bunch of content: tracks and cars. Our production schedule is such that we are putting them in as late as possible and that means making them free as downloadable content on Day One. [But] that is required content to play the game. We basically have designed the game to work with all that content no matter how late is coming in, in order to make the biggest game possible.”
It sounds like Turn 10 will be developing Forza Motorsport 5 until the game’s release on Xbox One this November. Just remember you’ll need an internet connection in order to play the game.