I have a CS200 and find that it is under recording distance. No sure on the speed reading. I have just changed the wheel setting from the one recommended in the book as when i measured the distance of one rotation I found about 1% difference.
This however is not enough as I was out by about 5%.
I did notice that at slow speed, ie when going up a hill at 10kph the speed would drop out and record 0.
This could also account for the low distance.
To try and correct this I have moved the sensor as high up the fork as possible, just under the brake, and will see if this helps.
Will keep you informed.