Corrected entry: When Taejo's sister Horuku comes to give the Grand Prix ticket to the Racer family, Speed racer laments they "don't have a car". BUT- when Horuko walks into the house, you can see the ever present Mach 6 in the background in what should be the dining room. They do have a car and modifying/upgrading it would be far easier and quicker than building a new one from scratch.
Zoddious
30th Jan 2010
Correction: That's the Mach 5. You can even see the red encircled yellow "5" on the passenger door right before Pops, not Speed, says "We don't have a car." The Mach 5 is a rally car, essentially street legal. The Mach 6 is a T-180. It would take much much longer to modify the Mach 5 because before you could turn it into a T-180 you'd first have to take it completely apart.
Phixius ★