If you have an iphone, consider using one of the ANT+ setups. I have the Wahoo Fitness speed/cadence sensors plus the heart rate monitor. It isn't terribly cheap, but it utilizes my iphone and I can't be far from my phone! There are lots of free apps for all of the different smart phones out there as well that use GPS to give you a ballpark idea of what you are doing on your bike. Good luck!