Track Builder Digital Speedo. The Hot Wheels Track Builder System offers the ultimate track experience for boys by allowing them to design, create and customize their very own track challenges. Using a variety of stunts, layouts and essential pieces that work together or connect to existing sets, boys can be the master of their own track. This Digital Speedometer answers boys big need for speed! As you go racing by, it measures your car's Hot Wheels Speed. Test out how fast you're going, try to improve your velocity or race a friend!