IMO, just buy a simple lap timer. You only need to know how consistent you are from lap to lap. You don't need a speedo either, they're just distracting on the track. Put a piece of tape over it and don't worry about it. Edit: actually as new track rider you shouldn't even be worrying about your times on the track. Work on the basic skills first then worry about your times. It will all come with time, trust me.