They could well also use speed sensors, but I wouldn't have thought it was necessary, as you can buy "standalone" sat-nav that just sits on top of the dash, and the delay on the speed system on my sat-nav has a 1-2 sec refresh rate, which would imlply that it was not getting "outside" help.
Just wish there was a mute button for the voice, as I don't go the way she says for the first bit of any journey (because I know my home town better than she does).