The stock wheels on U.S. models are either 406 mm (20 x 1.75) or 451 mm (20 x 1 3/8). Generally the folding models came with the 406 size, which is also the size used for BMX bikes. The 451 mm size is somewhat scarcer. Good rims and tires are available in both sizes, though the selection is much larger for the more common 406 mm size. The stock steel rims are very heavy and don't provide very good braking. The most valuable upgrade to these bikes is to replace the rims with lightweight aluminum ones, and to install better tires.