It is likely set for the wrong tire size. Some new bikes come with a 700x26 tire that isn't even listed in the owners manual.

Speaking of interference, mine was a pain to set up but not due to the unit. Something in the area of my stereo and TV interferes with the unit so when I was setting it up, it couldn't communcate between the sensor and unit. When I tried later across the room, it worked great. I can spin the wheel near the TV and nothing registers at all.