GT2 timing belt is teeth are designed for zero backlash which is perfectly suited to drive a 3D printer with the correct pulley. For the best performance you need at a minimum of 6 teeth in contact with the pulley at all times, this means at minimum a 12 toothed pulley and with the belt being fiberglass reinforced; the belt is more flexible than traditional steel wire reinforced and can wrap around the smaller pulley.
GT2 timing belt: 6mm wide, 2mm pitch, fiberglass reinforced rubber. Sold by the metre to suit all requirements. Orders are supplied in one complete length, uncut.