Emile Hirsch was very believable as Speed, and brought the right amount of boyish sincerity to the role. Also, the physical resemblance is very uncanny.
The M on Speed's car and his helmet, by the way, are reputed to stand for the M in Mach (as in Mach 5). That's the American story. The Japanese story (the original version) is that the M stood for Mifune, the family name in the Japanese Mach GoGoGo, and the character's name is Go Mifune (go here being a homonym for the Japanese number 5, which is the number on the car, of course. Wheels within wheels.
Christina Ricci, to us one of the proverbial women who could make a burlap sack look glamorous, is note-perfect as the perky, strong, and capable Trixie:
Perfect, as far as we're concerned. And she not only flew the Mach 5's spotter copter (just as she did in the cartoon) but she also did some dead brilliant driving. In other words: our kinda woman.
Her name in Mach GoGoGo is Michi Shimura, and the anime version had an M on her blouse, standing for her first name.