Didn't Ford do it with the DEW98 and D2C essentially? More often than not, it does't seem to work like that though.

 

Ford actually announced a "Mustang-based sedan" at one point ... what happened to it is a good question.

 

I think the "expense" of Sigma has been highly exaggerated.
The internet myth (continually propogated by some), that Sigma is very expensive because it is intensive in aluminum is completely false. There is nothing exotic about it.

Basically, Sigma costs several hundred dollars more than Zeta to produce - and virtually all of that added cost is in it's front suspension. Where Sigma (and Zeta) falls on it's face cost wise is on it's production volumes.

Imagine the cost savings if the next gen CTS, next gen Camaro, and all the ATS variants, shared an architecture and assembly line.

 

What I meant is that GM hasn't pre-announced it yet, to the best of my knowledge. Maybe "rumor" is a better word than "speculation".

 

Some recent comments by Nesbitt, and previous comments by Lutz, seem to support it.