Since the prologue sold 4.6 mil already i can say that its already a success.
A demo which costs as much as a full game can sell that much so we can say
its PART of the full game's sales.
If GT5 sells 6 million its a success since 6 million + 4 million = 10 million PROFIT!
Plus it can be heavily bundled in Europe so every PS3 sold in Europe will have a GT5 copy sold!!
European REALLY love their cars so i hope Sony capitalizes on this.
I REALLY don't see why GT5 will be a failure when compared to the previous games.

GT3 was bundled in NA, and none of the other GTs have. Simple.

Majority might have bought GT3 for the PS2 to experience GT in PS2 graphics. For GT4, it was released on the PS2 too, thus some might think they don't need to buy another GT again for the PS2 since they should almost be the same experience. GT4 might have appealed to only hardcore and racing fans while casual racing fans might not have bought it.

GT5 is the first GT on PS3 and in HD, thus demand will be very significantly much much higher. I can say safely that it will at least sell 3 million in the US alone.

Racing is a very broad genre and almost everyone have some inclination towards racing games. I myself am a more RPG/Strategy/Platform/Action kind of gamer but I will definitely buy GT5. This will be my first GT. God of War 3 is a possibility too but I am still not sure about buying it. But I am already all hyped for GT5.

Well Gran Turismo 4 didn't launch with online play, which had been promised for quite some time.

Playing games online was a major selling point for console games from 2001 to about 2005. After that it was just expected. If you didn't have it, it hurt you.

Gran Turismo 4 also released when the talk of next gen systems hit a fever pitch. Just mere months after the release of GT4, the PS3 was unveiled at E3 in May.

GT 5 will have a much better time in America than GT 4 did. With Nascar being so close to it now....it's a given.

Not only Gran Turismo,the whole racing genre.

Except Mario Kart, which has exploded this gen.  Mario Kart DS+Wii have sold almost as much as GT1-4 combined!

I think it should be noted that GT3 A-Spec was one of the first few games that earned AAA reviews on the PS2. As a result, the interest level and hype is probably much greater than we could possibly imagine. Bundling also helped a bit.

Perhaps there is a decline in interest in driving simulations over the years.

It's interesting how GT 5 is ahead of a certain game in the Americas preorder chart.

this game should be bigger than Christmas so do not worry.