9 speed cassettes are exactly the same width as 8 speed cassettes, so the derailleur moves the same total distance to shift 9 gears as it does to shift 8 on an 8 speed cassette. It is for this reason that the same wheels can be used for 8, 9, or 10 speed cassettes. In fact, a Shimano 10 speed cassette is a bit narrower than a 9 speed or 8 speed cassette and requires a 1 mm spacer to fit on the cassette body