We need for $C(x)$ to denote the car $x$ is a fast car.
I'm not clear what you're trying to conclude, but we cannot conclude $\lnot C$. We cannot conclude anything about how fast Dan's car only from the knowledge that his car is not a Porche.
We know Porches are fast, but other makes and models may also be fast, perhaps faster! And Dan may have a fast, "non-Porche" car.
In general: From $$P \rightarrow Q$$ $$\lnot P$$ we cannot conclude $\lnot Q$. That's a fallacy in reasoning: sometimes called "denying the antecedent".
The error is that:
- It's a misapplication of modus ponens, which tells us that $$P\rightarrow Q$$ $$P$$ $$\therefore Q$$
- or a misapplication of modus tollens which tells us that $$P
\rightarrow Q$$ $$\lnot Q$$ $$\therefore \lnot P$$