Even without Japan Gran Turismo 5 would outsell Halo 3.

Gran Turismo 5would outsell in Japan and Europe but Halo 3 in America...by a bit. Thus GT5 would outsell.

Halo 3 would win.

If people can look beyond the biasedness of their fanboyselves (is that a word?) they should look at some facts.

GT4 sold 9m in 5 years with an install base of 120m units. That means just 7% bought that game.
Its first week sales worldwide were 2.4m with again 120m units to sell to.

Halo 3 has sold 9m in 15 months with at that point 18m units to sell to. 50% bought the game.
Its first week sales were 3.8m worldwide, nearly double the opening week of GT4.

By the end of its 10th week on sales Halo 3 sold 5.9m units worldwide.
GT4 sold 5.09m by its 10th week. Meaning Halo 3 outsold GT4 by almost 1m by the end of its 10th week. Halo 3 has over 3 years left of sales before it is has had the same amount of time as GT4 on sale. Meaning Halo 3 sales will obviously increase by a few more million in 3 years. With the expansions coming more will be sold.

Pro-logue has been out a year and has sold nearly 3m. At the end of its 1st week it sold just 600k. Not even comparable.

So there is no question Halo 3 would win because it sold more then the last proper Gran Turismo game with over 100m less units to sell to and with 3 and a half years less time to sell the game.

To assume it will match the sales of A-Spec is laughable.

IMO. What you might forget is that when GT4 was released there were already many great PS2 games to choose from.  In the period Halo 3 was released, there were only a handful of good Xbox360 games so who wouldnt pick up a copy of Halo 3 cuz the fans strived for another good game.  During my breif period of owning a 360 all i had was Dead Rising and Gears, and i'd probably wouldve got Halo 3 cuz to me the 360 lacked alot of good games then. If you put the PS3 and GT5 in the same position as the 360 and Halo 3, but knowing that GT has a larger WW fanbase, im sure GT5 would do better.

 Funny, I never had a shortage of games to play back then so I think that argument has little merit.  Halo 3 was marketed to perfection and got rave reviews, it was a perfect example of how to deliver a hyped huge game.  GT5 will be lucky to hit 10 million if current PS3 exclusive sales are anything to gauge off.

@Hyruken

I hate to break it to you but the first installment of Gran Turismo on a Playstation is always more popular than its immediate successor. It would make more sense to compare GT5 to GT3. The install base of the PS3 will likely do nothing to hinder GT5's success due to the fact that Europe accounts for more than half of the series' sales and the PS3's sales trends in that region are similar to that of the PS2. Finally, take the fact that Gran Turismo is a system seller into account and GT5's sales are all but guaranteed to surpass those of Halo 3. I'd be shocked if Halo 3's LTD sales exceeded those of GT5.

 The problem is you say "has a larger fanbase", how do you figure that? Surely a fanbase is the people who buy the game?

To assume more people will buy the game is based on nothing but opinion. In contrast if you look at other succesful franchises your notice that they have not sold anywhere near as much this gen as they had last gen.

A way to disprove your theory is if you take the biggest selling game of the previous generation. Grand Theft Auto San Andreas. That game sold nearly 18m in 5 years. Sold nearly 4m in it's first week. 

The next installment GTA4 sold on PS3 5m in a year on sale. Sold 2.5m in it's first week.

Thus concluding that if the biggest selling Playstation 3 game of all time has sold nearly 50% less in its first week and sold over 75% less in total to its predassesor what chance does GT5 have of matching GT4 sales? None whatsoever.

Another important point is how well GT5 does review wise. There has not been a GT game with a score above 90% in almost a decade.

@Hyruken

Your logic is terrible, San Andreas was a timed exclusive and GTA4 was not. /GTA sales discussion Gran Turismo always sells better in its first iteration on a console. That's why GT1 & 3 were more popular than 2 & 4. Following these sales trends and the fact that the PS3's success in Europe (GT's biggest market) resembles that of the PS2 it's likely that GT5 will outsell GT4.

 We'll if what you say is true then you have just confirmed that Halo 3 will sell more. If GT games sell better in its 1st installment on each console means GT1 sold 10m in 11 years. That is basically 1m a year ratio. Seeing as Halo 3 is currently at 4.5m a year ratio = Halo 3 wins. Then we take GT3 which sold around 15m on the PS2. On sale for 8 years. Making it have a  1.85m a year average. Again not even close to Halo 3's average. In 8 years time i am pretty ceratin Halo 3 will have sold near to what GT3 has sold. And since GT3 in 2001 no GT game has come close to those numbers. Once again concluding that thinking GT5 will beat Halo 3 numbers = massive assumption which is very unlikely.

No your logic is terrible. GTA:SA is the biggest selling Playstation game in history. You said that games sell better in their 1st iterations which is complete bollocks. GTA:SA outsold GTA3.... The fact that GTA4 is a multi-platform game is irelivent. Your point is that because it is a high named franchise therefore it must automatically beat its previous sales. People then must rush out to buy the console and game. Which has not happened with any of the PS3 games released this gen. What you fail to grasp is that there are 20m PS3s out there. You assume that 50% of those owners will buy GT5 because it is that big a game. But even if that many buy the game it won't reach GT3 numbers, won't reach GT1 numbers and so on. More PS3's need to be sold to reach your assumptions and with sales declining by nearly 30% year over year that would lead me to believe demand is not as high as you try and say. For GT5 to beat Halo 3 sales means it has to elevate the PS3 sales by a lot. If previous franchises are anything to go by this gen then as i said your theory is unrealistic. Speaking from an analysts point of view i cannot see GT5 being anywhere near the sales of Halo 3.

Grand Theft Auto is one of if not thee most popular games franchises ever created with sales that are far superiour to that of the GT franchise. And if the 1st installment on the next gen consoles means that only 25% of PS3 owners bought a copy GT5 will not be anywhere near it. And that is the point you missed. If PS3 owners have not bought GTA4 which scored perfect 10's from most places in the same level of % as what 360 owners have bought Halo 3 then what evidence do you have to suggest it will sell more? None at all.