Eisboch wrote:

I have been told that factory GPS and most high-end retrofit systems for
cars also use a speed sensor input from the car transmission that is
separate from the GPS signal. It is used primarily to keep position
calculations going when the satellite signal is lost (tunnel) but it also
helps improve the accuracy of the position readings.


That is the claim for my wife's Toyota nav.

I don't think ours uses a "lock to road" feature - watching the marker
as we drive we were frequently 10 to 20 feet on either side of the road.
Also, it seems to take us "off road" 50 feet going down the driveway.