Get an iSport powermeter from www.ibikesports.com-I use a iSport GT and save the data on an Garmin Edge 500. The iSport does what you want and is the most accurate way to measure instantaneous % grade. I use it all the time (have measured up to 19.0% grades, real-time). This is even more accurate than post processing barometric pressure altitude data from my experience, for finding the peak grades of a ride (and it measure the peak grade for a ride also).