Forza 5 is the fastest-selling racing game in Xbox history, Microsoft has announced.
More than one third of Xbox One owners have bought the Turn 10-developed racing game, which launched alongside the console on 22nd November.
Microsoft has shipped 3.9m Xbox One units globally, so we're looking at around 1.3m sales.
Other Xbox racing games include the past Forza titles and the Project Gotham series.
The news comes as Turn 10 announced Road America, the first track add-on update for the game. It will be made available for free and will be automatically added to the Career events for the racing career. Also added are new Road America-themed events in Rivals Mode and in multiplayer lobbies.
The Road America booster pack includes five cars. Out today, it is a free bonus to Forza 5 Car Pass owners and, Microsoft said, is in addition to the six monthly DLC packs that Car Pass owners will get through to May 2014. The five cars are the 2006 #2 Audi Sport North America R8, 1995 Chevrolet Corvette ZR-1, 1968 Ferrari 365 GTB/4, 1971 Ford Mustang Mach 1 and the 2009 Mercedes-Benz SL 65 AMG Black Series.