Does the weight of a toy car affect its speed?

Does the weight of a toy car affect its speed?
It depends on a lot of factors. The classic question which will fall faster, an apple or a bowling ball comes to mind. When you are talking about a car, it is no longer vertical and not directly correlated to the pull of gravity. If both have identical components, and going downhill, the heavier car will have more momentum behind it (Potential Energy). Along the flat, the heavier car will have more friction. Uphill, the heavier car will have a bigger energy difference, and more energy (more work) will be required to make it go up the hill.

In short, the heavier car will win going downhill, that's it. And it will hurt more when it hits.