measure the most worn area ie wide glides wear front tires more on the side so measure there, on the rear of an ultra it will usually be fairly central, 1/16" is bin time, there are also tread wear indicators on em best way to find them is look on the side wall for a small triangle that is the locator for the TWI. alot of the ultras here in Australia are getting over 20,000km or so (about 12 or 15000mls)