This prediction is based on frame-by-frame viewing of the trailer, revealing that while the Mach 5 is recognizable and present (as in the icon above) about half of the race footage has Speed driving a single-seat "more suped-uppier" vehicle emblazoned with a 6 on the side instead of a 5. Presumably the Mach 6 is what he drives in the climactic "big race" at the end.
Also, that's how Hollywood thinks.
EDIT: I wouldn't care really, except that the Mach 5 is the star of the show. Speed, Trixie, et al. (with the possible exception of Racer X) are all just people who also happen to be in it. You'd think the Whatsahoosis Brothers of all people would get that.