I was under the impression, though I may be wrong, that you do not need data once the route has been calculated. After that it uses GPS just like any other sat-nav and it only needs to use data if you go off course and it has to recalculate.
I only use it in my motorhome as my car has built-in sat-nav, so won't be using Google for a few weeks but the next time I use it I shall try switching off data as I set off.
Possibly some techie on here can confirm if my understanding of this is correct?
I used it for the one and only time for a longish journey last month and it was excellent. I didn't bother about data use as I'm on an unlimited data plan.
However, I shall be going to mainland Europe where data use is more expensive, so I too will eventually need to know if data is used for navigation.