i would say more like: How bad GT5 sales will be compared to Forza 3 proving that people are tired with game that is still a port frm psx? cause i think this time Forza not only still do better than Forza 2 as usual, but also the gap between GT5 and Forza 3 will be less than 1.5-2mln.
Forza 1 - 1mln LTD
Forza 2 - 4mln LTD
Forza 3 - 5-6.5mln LTD probably
GT5 - maybe 7mln LTD
to people that says sales of Prologue are some indication that GT5 will sell gazilions on the launch day:
GT5prologue have seen several rereleases in japan(they don't have dlc so if you want newest cars/tracks you have to buy another game),
it was heavily bundled pretty much everywhere and at some point it was the only bundle you could get,
it was budget title from the start,
ps3 have much smaller userbase and huge chunk of GT series sales were always from casual gamers who pretty much never cared about the franchise they wanted only a good looking racing simulator with a console for reasonable price,
it's pretty much the only good racing sim on ps3,
if it wasn't for that it wouldn hit even 2mln copies,