There is little reason to believe GT will not make 8-10M.
It releases on a pretty similar term as GT3 did. GT3 never sold during the big Ps2 userbase, but rather when the Ps2 base was 10-40M, and also some when it was 40-60M (GT4 released at around 55-60M I believe).
GT5 is going to sell most of its copies during 25M(30?) to 35-40M installbase, with some units in the 45-55M.
Other reasons to believe that GT will sell well, is that we do, this generation, see that fewer games tend to sell a lot, but the ones that sell a lot sell more than they used to.
So GT5 will probably be the best selling Ps3 title, pretty much ala Halo 3, which will probably sell about as much as GT5.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS
