On the track with tires its 1/5th but my buggy and truggy have been round. The other track in for anything smaller but most of us have 1/8 or 1/10. But they dont mind what track you use with the 1/8th or below. The only thing we dont do is run the 1/5th on the track with the piping around it, as that makes big ruts and churnes it up. Hope you understand and get that if not just ask.