What is this speed sensor Ted? I often use mine on a roller but have to rely on another computer. I do turn the GPS off to avoid it scanning for satellites all the time.
My cadence sensor also has a wheel speed sensor built in - so on my road bike you have a magnet on the crank for cadence, and one on the back wheel for speed.
Picture here - http://www.cyclesurgery.com/pws/UniqueP ... tAodMwIA7w
- cadence sensor is the flat side of the main unit, speed sensor is via the adjustable arm. They've helpfully let the two magnets (crank and wheel) become "as one" for the photo.
Build it like you owned it!