The SC-6500 can only be setup for a single bike, whereas the SC-6501 can be setup for 4 bikes. The SC-6501 also has a facility to input the Odometer miles when setting up. Also the SC-6501 has dedicated 7 & 8 speed displays while on the SC-6500 the 7&8 speed gears are shown on 9-speed display. The SC-6501 also allows for a smaller wheel size range starting at 1300mm instead of 1700mm.