We've seen the GT5 unboxing with some Gran Turismo 5 Signature Edition pictures, and today GamersMint.com (linked above) confirms via amar212 at http://www.gtplanet.net/forum/showthread.php?t=132738 that Gran Turismo 5 has an optional 40-minute 6.4GB PS3 HDD installation with the US game being reported as 19.4GB (20,882,698,502 bytes) with the EU version coming in at 13.72GB in size.
To quote: "The initial install size of Gran Turismo 5 on your PS3 HDD is going to be 6.4 GB. He also revealed that it takes about 40 minutes to install.
However, interestingly, he says that the game will continue to install and decompress data throughout the course of your playtime and the actual disk space taken by the game on your PS3 will be a lot more.
Kindly note that this installation is optional and you can play the game without doing this. It only requires 256 MB of your HDD if you decide against doing it but it's highly recommended that you do if you plan on spending days with the game like we do."
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