Let's look at Japan.
The biggest selling game in Japan this gen is FFXIII with 1.86m units. It did 1.4m in it's first week there. That is the highest opening for a game there this gen. However if you look beyond that game your see no other game has broken 1m sales on PS3 in Japan let alone in 1 week. In fact the next best selling game is MGS4 with 800k. To get into the top 10 sales this gen on PS3 in Japan you only need to sell 400k.
Now then let's look at games in Japan that sold well last time around. Winning Eleven. One of the biggest franchises in Japan, guranteed to sell well no matter what. 4 of the top 20 biggest selling PS2 games in Japan were WE games. With total sales of 4.46m. Total GT sales in Japan on PS2 was 3.1m units, meaning WE sold more then GT games in Japan last gen. This time around once again WE games make up 3 of the top 17 games on PS3. As far as i can see the series has the most titles of any series this gen in the top 17. However the highest selling WE game this gen has sold just 450k, amd 223k of that was in it's first week. 710k less sales then the top selling WE game last gen.
If people think GT5 is going to sell 1m in it's first week in Japan then i think your dreaming.
500k-700k is what i think it will do in Japan first week.
Then comes europe. It will do 1m in europe for sure.
US i think will do the same as Japan and will be between 500k-700k.
Total first week sales will be between 2m-2.4m. Meaning it will have sold less first week then GT4 did.
