Some of the things that slow balloon cars down, that you have to overcome, are their own weight, air resistance, friction, and inefficient use of the air escaping the balloon. Reducing weight, minimizing drag, cutting friction and improving nozzle air flow will all help make your balloon car go faster.
Examine your car carefully for any opportunity to shed as much weight as possible. The force that pushes your car forward is equal to the mass of the car multiplied by the acceleration of the car. Therefore, when you decrease the mass, the acceleration will increase and the car will reach a higher top speed before the balloon runs out of air. Use lightweight cardboard or balsa wood for the frame, and attach parts with as little tape and glue as possible.
Drag, or air resistance, is important even at low speeds because the balloon car is so lightweight. When you reduce the drag, less of the force from the balloon is lost to pushing against air and more is given to increasing the speed of the car. Make sure the front of your car doesn't have any large flat surfaces. Instead use curves or wedge shapes, and give the car a low profile so it will more easily move through the air.
Cut the Friction
Friction from the moving parts of your car, namely the wheels and axles, also takes energy away from the balloon and from the speed of your car. Use lubricants to reduce friction. Balloon car axles are often made from wooden skewers, so common oil-based lubricants may not work very well. However, you can lubricate with graphite, which will not soak into the wood. Sprinkle or rub dry graphite onto the axles and into the axle tubes, then give the wheels a spin to spread it around.
Improve the Nozzle
Improve the speed of your balloon car with a nozzle to make better use of the balloon thrust. Nozzles increase the speed of the air leaving the balloon, which then produces more thrust. A nozzle will also keep the air pushing smoothly in one direction without losing energy to the rapid flapping that occurs when air exits the rubber opening of the balloon. A balloon car's nozzle is usually a plastic drinking straw attached to the balloon with an air-tight seal. Tape, glue, silicon, or other sealant can be used. Experiment with different straw diameters to find the best size straw for maximum speed .