So not to ruin the thread by quoting a massive long post i thought i would put a new one to respond to this time thing.
Right there is one critical flaw thing about this whole GT3 selling loads with a far smaller install base etc..Some seemed to have ignored it while on their little crusades.
When GT3 came out the PS2 wasen't even released outside Japan.....GT3 was a launch title for most territories. Your comparing sales for one region, obviously the attach ratio will look better by doing that!
If you want a better indicator of this time thing then look at the first year on sales after it's release. In that Year PS2's total install base was 28m units....Only 7m short of what PS3 has sold in 4 years on sale.
So the point that some are making is GT5 can sell like GT3 if the pattern on the PS2 repeats itself. In that the console is sold at the same rate as the PS2 was.
Again not going to happen.