Hmmmm...
That makes sense regarding the different kinds of cars being used for different races. I didn't mind the differing designs, especially since the classic Mach 5 is what we see the most (and I'm still overjoyed that they kept its design completely identical to that of the cartoon; take note, Michael Bay). I just wish they would have kept a #5 on the side of the car across all of the different incarnations. That one little detail would have made a world of difference in my book.
I actually got to the theater about ten minutes late.

I got there as Spritle was saying "Do you think he's gonna do it!?" in reference to Speed possibly beating Rex's record time on that track. I watched the first seven minutes on SciFi Japan when I got back (talk about convenient happenings), so I've really only missed a minute or two of the film. After watching that I was kind of confused as to why Speed was
driving the Mach 6 at the beginning of the movie, but your explanation makes sense and I do think it works within the film. Just agh...
give it a #5 and all would be right in the world!