I think GT Prologue selling ~2.5M is fairly close to what GT5 will do, 3.5M tops.
I don't think it's still the huge franchise it used to be.

The install base just isn't there for a huge seller, and Forza 3 with a cheap 360 will keep it from moving much for hardware.

I doubt iot, there aren't that many racing fans with the userbase at this point

If they do pricecut and and bundled it, it has a chance to pass it.

You think all 14.8 million copies of GTA 3 A-Spec sold right away?  It took years.  The PS2's install also increased far faster than the PS3's install base is.  By the time GTA 3 sold the 14.8 million units the PS2 had over 65-70 million units sold.

Some people say Ferrari Challange is good if you have wheel.

But for me any racing slower than Wipeout HD is boring :D

WTF man, both Super Smash Bros. Meele and Halo 2 topped 7 million and XB/GC not even managed to sell 25 million, franchises like GT keep doing strong, plus nobody cares about Forza so stop the X360 fan ranting.

I'm going to say it will sell from 8.5 to 10.5 so it could it might not but it will sell well...

You're basing your projection on a lot of "ifs", not facts.   The facts, as we know them today, do not suggest a 10 million seller.   projecting 10 million requires a sales model that depends on factors beyond our knowledge and calculations.

Ok lets see how close GT5 can be to 10 million, it's real close.

Let's say that Gran Turismo 5 will release on January of 2010 (I don't know if that's true but it might be around that date) the userbase of the PS3 (this is the minimum of what the PS3 could be) should be around 3.5 million in Japan, 10 million in America and 14 million in Total Others (Basically figured out what the PS3 sold this year in each region and copied it for next year) so that's a total of 27.5 million.

Now lets say something happened and the PS3 was discontinued next year and only sold 27.5 million for the generation, now out of that 27.5 million how much should be sold in each region.

So gran Turismo 3 and 4 both sold 10% of the toal PS2 sold in Total others, so that's 1.4 million for GT5 in Total Others, and 14% of PS2 owners in America owned GT3, but for some reason only 5% own GT4, so let's split it down the middle and say 10% PS3 owners will buy GT5 so thats 1 million, now for Japan there was 8.5% of PS2 owners owned GT3 and only 5% own GT4 in Japan so lets say 6.5% of PS3 owners for Japan will buy GT5, thats 220K (looks way to low I know). So the total should be around 2.6 million (this is the minimum number GT5 will sell from what I see, it should sell alot more then that total).

Seems very low but there's alot of factors I didn't take into account, like a PS3 price cut, a very possible bundle, the release date of the game and obviously the PS3 isn't going to end this generation with 27.5 million, it should be able to sell atlease 65 million for this generation. So I figured out that 2.6 million of 27.5 million PS3's, should be around 6.1 million if the PS3 sold a total of 65 million, not to mention the bundles and the PS3 price cut's in the future, then it should end with around 9 million according to what I got.

(Again this is if everything stays exactly the same as it is now for the rest of the PS3's generation)

@Gilgamesh: If you look at the final consoles' LTD sales, the attach rates will be horrible. San Andreas sold under 15% of the current PS2 install base, while GTA IV is currently at ~28% of the PS3's. Also, I highly doubt the PS3 will make it to 65mil.

I'll repeat what I said in my earlier post once more:

• GT3 managed a 30% attach rate in Japan, by selling 1.7m copies to 5.6m PS2 users.
• Over 6m copies of GT3 were shipped outside Japan (8m inc Japan), when the PS2 userbase there was ~17.5m (VGC numbers).
  
• Over 9m copies of GT3 were shipped outside Japan (11m inc Japan), when the PS2 userbase there was ~35m (VGC numbers).
  

(I'm aware shipped > sold, but VGC doesn't have sold on here)

With that said, this is what needs to happen for GT5 to hit 10mil:

Obviously, the game needs to be as big of a hit as GT3 was. If it can keep its high attach rates it had with early PS2 users (25%+), there won't be a problem. Also, the PS3 needs to sell at least 45m consoles. If Sony fucks up in 09', there's a chance they won't sell enough.

My guess the split would look like this:
~1.2m Japan, ~3.4m Americas, ~5.4m Others

Yeah, it might be somewhat of a longshot, but not entirely out of reach.