The likelihood that most of the study participants were unaware of the actual commonness of their make of car is irrelevant to the argument. The argument is about whether different cars are more common in different regions, not about people’s car stats knowledge.

The researcher concludes that a hypothesis is true merely based on a premise that supports the hypothesis. This is a flaw because a hypothesis can have some support and still be false, for example if the same evidence is consistent with multiple explanations.

Whether or not the subject pool comes from a variety of regions isn’t relevant, because the argument’s evidence just depends on most participants overestimating how common their car is.

The researcher does not use any premises that contradict each other in this argument.

The argument doesn’t apply a generalization to a particular case. It’s more that the researcher is trying to make a generalization about car distribution across the nation based on a different generalization about how common people think their make of car is.