Then it's up to you to piece your car together, combining different Lego bricks from each of the car sets. And there are a couple of things you need to think about when you're constructing weight and balance. You'll want to stick to having a balanced car, which means don't put a lot of bricks on one side of the car. This will keep you from veering off to one side. The weight of the car depends on the chassis and the number of bricks you put on it.
Heavier cars go faster and are easier to steer, but take a longer time to accelerate.
Lighter cars don't go as fast and don't turn as easily, but accelerate more quickly. In MK terminology, think Donkey Kong or Wario versus Yoshi or Princess. Also remember that certain types of cars will race better than other types on a particular track. While much of the action takes place in Lego themed environments, the environments hardly reflect this. A few scarce background objects appear to be constructed of Legos, but otherwise High Voltage Software has created the 3D courses from the ground up.