GranTurismo you name suggests you are a fan of the game, and i would agree it is a great franchise with excellent games, but combining GT3, GT4, GT4prologue and the concept thing which was basically GT3prologue doesn't mean it is one game.
GT3 itself has sold slightly over half of your number, that is the maximum number a GT game has sold, and by the sales of GT4 i would guess peoples interest is waning.
GT5 in my opinion, (dependant on how the PS3s userbase grows and how good it is in the consumers opinion) will sell between 6 million and 18 million....the 6 is because the PS3s userbase is small and GT4 hasn't sold spectacularly and the 18 because it should continue selling from release (summer/autumn 08 ?) for at least a year and a half, and because even i still think the PS3 has a chance at selling over 60Million in it's life, so a 30% attach rate seems possible.
and again IMO, my personal thoughts for where it could come in that range (6-18M) would be about 9M, because i think it will be about as successful as GT4 despite the huge differences in userbase. (although fans of the game would follow the system so strict %ages can't be used (ie 8M on a 100M userbase doesnt mean 800k on a 10M userbase).... i think roughly 3/4 of people who owned GT4 on PS2 will end up with a PS3 and GT5
