I like that experiment. Carbon dioxide is a gas that is pushed into soda, so it starts coming back out of the soda as soon as you open it. I'm guessing there's a difference in how far the soda car goes if you shake the soda before you open it, or if you don't shake the soda. Any soda with sugar in it will be sticky, so I hope you have a place to test your cars where that's ok.

The fizz that bubbles up when you crack open a can of soda is carbon dioxide gas (CO₂). Soft drink manufacturers add this tingling froth by forcing carbon dioxide and water into your soda at high pressures—up to 1,200 pounds per square inch.

Carbon dioxide is the gas that makes soda fizz. I don't know what a soda car is, but if it's a car that uses the bubbling of soda to propel itself, then the more carbon dioxide in the soda, the more propulsion the car will have.