Spychangers seem to be about the same size as hot wheels cars (which was a good selling point) and the wheel rotation seems smooth. The only problems I can see compounding use of them on the speedy hot wheel tracks is being plastic the weight and durability will be lower. So useing collision style tracks with the metal cars could damage spychangers. However I think a spychanger could move faster on a motorized track than a hot wheels car because the lower weight means a lower amount of resistance to keep it in motion. (modern hot wheels tracks use motors as opposed to the elevated chair set up.) It's something I've been thinking about.