I'd just start by making sure they are both zeroed. Go to the watts line by hitting mode, hold down select til it flashes, hold select until it reads zero, release and press again til it turns solid. I have had 3 powertaps and I could zero one, put on another wheel and it read 12 watts differently. Saris seems to think this is normal and is one of the reasons I feel more comfortable with the SRM where you can determine the slope and set it yourself.