well let me state that clear! GPS is very precise as long as you use a device that is capable of doing the calculations fast enough, my other mobile gps doesn't really give you precision! constant speed is fine but accel. is very poor as it triggers about every second or so. with 2m/s you already get a magnet triggered once per second that's just as good as my gps... with 10mph you get it triggered twice and so on... there was one with 8-16 magnets-disc for high precision, does anybody know the name (google didnt help), for alt. you need gps or a better bike computer (there are some with alt measurement and stepping frequency)