I'm confused, how does getting the speed directly from the car improve directions and heads-up display of speed over getting the speed from the GPS unit?
The big advantage over built-in car navigation systems vs hand-held GPS units is that they measure speed and distance far more accurately via the wheel sensors. If you live in a city with a lot of tunnels or GPS dead spots, this can make a real difference.
I would imagine the same information applies. If you can blend the GPS and the wheel sensors, you would get the most accurate reading, because wheel sensors can become inaccurate through fitment of different size wheels and/or tyres.
Speed readings obtained from a GPS unit is never going to be as accurate as the reading derived directly from the car, because GPS locations are only accurate to about 10ft (IINM).
GPS extracts velocity data from the doppler shift of the signals, not by subtracting successive locations, so the 10ft accuracy doesn't come into play. My experience with GPS velocity data has been that it's more accurate than the average car's speedometer as long as you have a decent fix.
Of course, having poor signal in urban canyons or tunnels is the sort of thing I didn't think of, and that's where getting data from the car would really be handy.
It depends on a lot of factors, but with GPS there will always be lag and incorrectness. For inner-city driving, the GPS positioning might be really accurate but I doubt the speed calculation would be accurate, due to the constant acceleration, deceleration, and turning. The car's accelerometer will always be more correct.
Plus, my Galaxy S Vibrant has the worst GPS ever released, so I'm very frustrated with not having reliable navigation.
