From the physics thread:
That depends on what you compare the sales figures to. If its GT5 then no the FM series will not reach those levels, however if you compare it to any other racing title then its sales figures are more than good enough.
Even in comparison to GT its hideously difficult to even start to see if Forza is closing the gap or not, given the differing release cycles the two series have.
However just for fun we can given it a go.
In terms of raw units (to date):
FM2 - 3.99 million
FM3 - 5.33 million
FM4 - 3.88 million
GT5
- 4.11 million
GT5 - 8.65 million
Now if you compare single titles then yes GT has them beaten, however if you look at combined sales on the current gen platforms then its a lot closer:
GT5
and GT5 combined levels = 12.76 million
FM2, 3 & 4 combined levels = 13.2 million
Add in 'side' titles such as Horizon (1.12 million) and GT
SP (2.91 million) and we get:
Forza - 14.32 million
Gran Turismo - 15,67 million
I'd say those figures make future titles for both series look like a good investment for Sony and MS respectively. Nor do I think you can say what ever MS's sales plan was for Forza, that it didn't work. No other sim series has even got even close to the sales of GT and for that alone its certainly worked.