I just downloaded an app for my android cell phone called My Tracks its made by Google and its free. I tracks you path via gps, but the cool part is it graphs your speed and altitude against time and lays both graphs on the same time scale. I tried it while driving and it was right on with my speedometer, and as the car went uphill so did the altitude. I dont know how accurate the altitude was but the speedo was right on. The next time I go to the flying field you can bet I will have a hand full of different props and the phone will be mounted inside my plane.