@Kanariya
I didn't know that PD had a sales figure link in their website, good find there.
So, by my calculations, GT5:P sold 3,67M acording to PD. Maybe the difference of 0.62M from the data on this site is because that there is a lot of shelved copies, but i seriously doubt it, that is a very big difference just to be shelved copies. So probably, those are the sales figures including the downloads on the PSN, but at this point, i'm just speculating.
Even so, the fact still stands that GT5:P didn't outsell Forza 2, although I would never expected it to, it's a full game against a Prologue.
What struck me the most was the huge loss of american public from GT3 to GT4, it went down from 7,14M to 2,97M, I believe that Forza must have had some influence in here, and the fact that GT4 didn't offer anything really new over GT3 must have played a decisive factor as well.
So, in order for GT5 to be succesful, it would have to continue garnering it's european fans, which is almost a fact that it will, but it would also have to make some american fans to come back to the series, and that will be the biggest challenge IMO for GT
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