Someone seems to have figured out how characters were balanced speed-wise. Basically, every character specializes in certain driving roads, which gives them a boost equal to 0.2 more in Speed than what is visible on the stat screen when driving in those roads, either Smooth (normal road, possibly metal and ice road), Off-Road (dirt roads, sand, maybe snow), and Water (on water).

The featherweights (Baby Peach, Baby Daisy, Parabiddybud, Swoop) and super heavyweight (Bowser) do not have specialists. Their standard visible Speed stat already accounts for driving faster naturally on all types of road. On the same account, due to the way the game shows their stats in certain vehicles, the same is probably true for their Handling as well.