It doesn't work that way for me, I'm usually behind. Plus I find that subconsciously I'm trying to beat it, so I become frustrated. Stupid I know, but there you are. Mind you, I don't think I'd want to do 300 miles non-stop.Dont think its possible on new TomToms.
just calculates on known max speed limits.
one more nail in TomToms coffin....user interaction is bottom of their list of priorities.
it was only public demand on their forum page that got co-ordinate based route planning reinstated on the GO 50/500/5000 range.
Over a non-stop 300 mile journey i can gain around 15 to 20 minutes over TomToms estimated time.