Yes Garmins (i have no experience of others) do this.

It would be interesting to stop in the tunnel and see what happened when the car didn't re-aquire satellites when the sat nav assumed it would, presumably the sat nav assumes the speed entering the tunnel is maintained throughout?

I did wonder about that, but my speed today had to be pretty constant due to other traffic, so it wasn't obvious one way or the other.

Excuse the slight topic drift, but I've long wondered why some signals get through tunnels, others seem to do OK in vertical shafts (e.g. lifts) and others disappear altogether.
While driving and listening to FM radio (not DAB, as Spain doesn't seem to have it yet!) I lose the signal in road tunnels, yet a mobile phone call continues uninterrupted. Sat Nav continues though sometimes in some kind of reduced functionality (I use Google Maps on my phone, which is set to use phone signal or GPS or both).
However, I noticed when travelling in the lift that used to serve my neighbourhood (it ran vertically through a hill from the bottom of our road to the end of a pedestrian tunnel below) that the lift operator's FM radio stayed on all the time, yet my mobile phone signal (analogue until about 2000 then GSM) was completely lost.
Can any passing physicists explain?

Edited by Bilboman on 02/09/2019 at 22:47

Built in sat navs use the car's sensors to estimate position whereas simple gps devices should estimate using the speed you had when the gps signal was lost.

Most sat navs have a speed input from the car so don't need continuous signal - they also make assumptions if you have a route set up, they assume you stick to the route until the GPS position shows otherwise - this is visible if you ignore instructions to turn off a motorway or use a new road that's not on the mapping.