Figures of speech in Garfield

Not exactly. Mario Andretti is a retired race car driver. http://www.andretti.com/

But the driver in the film isn't excatly the Mr. Andertti himeself, right?

That is correct. There has been a convention in American English for about 30 years or so to describe someone who drives too fast as Marion Andretti--i suspect the use of the expression will die out as people begin to forget Mr. Andretti.

So synecdoche is correct then?

No, not really. You need to revisit the definition of synecdoche. In this case, although i cannot give you a precise linguistic term, the use of Mario Andretti is referential, ironic sarcasm. Mario Andretti became a by-word for a successful race-car driver--and therefore, somebody who drives very fast. One might ask someone, sarcastically, if they think they are Mario Andretti, the implication being that they are driving too fast.

This is similar to synecdoche, but not precisely the same. For example, to use "hand" to mean a hired man, such as a sailor or a cowboy, is synecdoche, and an example of a non-intuitive term which enters the language to mean a hired-man, and becomes intuitive through usage. Marion Andretti will not be intuitive to anyone who does not know who Marion Andretti is (was? I don't know if he is still living, although Marion Andretti, Jr. almost certainly is). That is why i've said that it will probably fall out of use as people who don't know who Marion Andretti was begin to outnumber those who remember him.