How do you compare the speed of hot wheel cars and match box cars with only one variable?

My daughter is doing an experiment on hot wheel cars verses match box cars. Her question is "Is a hot wheel car faster than a match box car?" She can have only one manipulated variable (assumably the cars).How do you compare the speed of hot wheel cars and match box cars with only one variable?
First of all, it should be more like this. What is the effect of toy manufacturer on speed of car. Second, I will gladly answer your question. Try setting up a ramp of books and then time the cars. If you do them at the same time then they could hit each other and ruin the experiment. Record the speed of how long it takes for the back wheels of the cars to go from the top of the book to the bottom. And Third, what grade is she in because that is extremely simple and I could give you an experiment setup.How do you compare the speed of hot wheel cars and match box cars with only one variable?
Try experimenting on the weight of the car.