Wow, what a lot of interesting comments!

Slow search:
Thanks, I will try to turn on logging.

Inertial navigation:
Isn't there something remotely related there already? On a junction, the maps rotates as if assuming I am following the route, even when I am not. It takes a few seconds to figure out I am not and correct itself.

DrYak:
Whoa, that's a lot of new info! The speed bias for example would explain why every single car I have ever driven has always shown I was driving faster than the GPS was indicating.