If you don't press start a movie will come up and in it homer is driving the monster truck on the first level. How does that work?

i don't know

but it is probably

1. A cheat
2. Only a clip for the beggining
3. They swaped the cars and forgot about it on the start
4. A mistake

---they are the most probabal reasons---

its a demo they changed it around.

You have to get 100% on the game (all races, all cars, all bees, etc) then you do a cheat ( check the section on that on Neo) then when you get to a phone booth, press L1, then you can play as all the cars (including secret cars, like the mentioned Monster truck) on all the levels. Enjoy!

(This works, though I didn't believe it at first)
The cheat is X,O,X,O (While holding L1 and R1 at the options screen)