[dcb]

Using both vss and accelerometer might be the most robust actually. Either one by themselves would have no way of knowing if you are climbing a hill or being affected by wind (though you need a wind sensor for that one).

The vss is a little chunky, but pretty accurate, since it is limited by your CPUs ability to measure time (i.e. within a few microseconds). If it is your typical 'murrican car with 5000 pulses per mile then @60mph, if you can measure 10us, then you will get 83 samples per second where each sample is accurate to 1 part in 1200, slower = more accuracy but lower sample rate.