Hello and welcome to Science Buddies,
It seems to me that what you are trying to do is to race cars of different mass down an incline and then make them of the same mass and try it again. Now assuming that you will race the cars down a plane downward incline, all you will need, once you have conducted the experiment is the knowledge of the force of weight and friction. A simple way of explaining the physics of this experiment is that the bigger objects have more force to overcome friction.
You can read about bodies on incline and the effect of friction over here:http://hyperphysics.phy-astr.gsu.edu/hbase/mincl.html
It is important to remove a misconception here. More massive objects having more force does not mean that a iron ball will reach the ground before a plastic ball of the same size when dropped from the same height. In a theoretical world of no friction, all balls and all cars regardless of their mass will reach the end at the same time. But because we live in an imperfect world, full of friction (no matter how hard we try), the iron ball and the heavier car finish first.
A deeper analysis of the role of friction is here:http://galileoandeinstein.physics.virginia.edu/lectures/gal_accn96.htm
The two different ball experiment and it explanation:http://galileoandeinstein.physics.virginia.edu/lectures/gal_accn96.htm
For an added challenge you can also actually calculate how fast each of the car should go using kinematic formulas* (*with the no friction assumption), but you would need to know the mass of the cars, the height of the incline, and the angle of the incline to the ground.
Here is the non-frictional physics behind what this experiment is about: http://physics.bu.edu/~redner/211-sp06/class04/incline.html
(Although it is a bit math heavy for this level, its the best I could find)
Basically if you look up incline physics, you would get a whole lot of information to cite.
I know this stuff can be complicated, so let me know if you have any questions.