Is there any pedometers that measure distance while on a treadmill?
Yes, there are. Horizon Fitness sells pedometers that will
measure the distance travelled while running on a treadmill.
A pedometer measures stride, assuming a certain stride equals a
standard amount of distance walked. If you slowly shake a pedometer
up and down you will fool it into thinking you've walked so many
steps. So, yes a pedometer will work on a treadmill.