Sim racers are pretty friggin' big in Japan.
The GT series is pretty big over there. Ridge Racer 7 has sold around 125k units on the PS3. RR6 on 360 has sold 72.8k, and is the 3rd highest selling 360 game in Japan. There are alot of factors that make it very difficult to predict where Forza 2 will end up.
Factor 1: Forza 1 didn't have alot of exposure in Japan. This can translate into a meaningless brand name (but for that matter, BD didnt have a brand associated with it, nor Dead Rising, or half of the major selling 360 games).
Factor 2: There are no major racing games to compete with. Motorstorm is long gone and dead, and so is RR7 for PS3. With no racers coming up, Forza could fill a huge void. Only TDU is the most recent racer out there.
Factor 3: This game is the finest racing sim ever created (ever play Forza 1? Far better than GT). If Famitsu gave it a 36/40, thats pretty dang good.
Factor 4: With Trusty Bell coming out merely 2 weeks afterwards, with quite a few projected new adopters for the 360, they could pick up Forza 2 as well.
Forza is a TOTAL toss-up. One one hand, it could do as low as 40k. On the other, it could nearly topple, or beat BD due to the obvious lack of next-gen racers. TDU is upto 20k, and that's a friggin' budget title (incredibly good game tho).
Range given: 40k (10% chance) upto 250k (10% chance). I'll go ahead and say 95k based on the following factors: The game will launch with sales of 52.5k or so O/W, drop to around 20k the week after. When TB launches, it will manage to hover in the 5-10k area for awhile. It's anyones guess as to how it sells systems. I'll put Forza in the "major system seller" catagory of which N3, Dead Rising, DOAX2 and DOA4 were in. It will move around 15k systems over it's lifetime. 7k first week (nearly beating, or even beating the PS3), then maybe 6k next week before TB takes over. But again, it could be the "thing" like the BD/LP combo we had in Christmas, and manage to move 50k-100k systems along with TB, or better. It's possible. It's also possible the sales only spike to 3,750 units too.