Forza Horizon car pack will drop monthly post release, and an expansion pack is planned for December, according to Playground and Turn 10.
The packs can be purchased individually or through the Season Pass through the marketplace. Basically like you can already do with Forza 4.
In the game, players can also earn treasure maps and popularity spikes. Treasure maps allow you to find all the hidden items – rare cars etc. – and popularity spikes work similar to XP.
The expansion will land on December 18 and Forza Horizon will be available on Xbox 360 October 23 in the US and October 26 in the UK.
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