Don't forget recumbents! They use both 406 and 451 with 406 being the more commonly available wheel/tire combo. Keep this in mind when you go to shop for wheels and tires, look to the recumbent specific dealers for better selection, price and stock. Two good sources for 20" wheels and tires are www.hostelshoppe.com and www.poweroncycling.com.Kerry Irons said:Go to Sheldon Brown's web site and go to the tire size discussion. I can't remember which, but one of those rims is for performance tires (what you want) and the other is for kids' bikes.