The past Gran Turismo's have sold like crazy, I know people that picked up the PS2 just to play GT3 and GT4 and I know people that are planning to pick up a PS3 just to play GT5. Take this into consideration, no GT game in the main series has sold below 8 million copies. If that is not an indication for the success of GT5 then I don't know what is.
While it can be argued that since the PS3 will probably have a smaller user base than the PS and PS2, consider than the Xbox360 has only around 10 million consoles sold and Gears of War has sold over 4 million copies, it is possible for GT5 to do easily as well as that, considering this game will validate purchasing a PS3 as you can show your friends how good the cars look.
In the end, I think this game will be able to push 6 million copies over its lifetime, 2 million lower than the lowest the series has ever sold, which might even be a low prediction but it all depends on how well the PS3 continues to sell.
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While it can be argued that since the PS3 will probably have a smaller user base than the PS and PS2, consider than the Xbox360 has only around 10 million consoles sold and Gears of War has sold over 4 million copies, it is possible for GT5 to do easily as well as that, considering this game will validate purchasing a PS3 as you can show your friends how good the cars look.
In the end, I think this game will be able to push 6 million copies over its lifetime, 2 million lower than the lowest the series has ever sold, which might even be a low prediction but it all depends on how well the PS3 continues to sell.
Sources:
http://www.vgcharts.org/worldtotals.php