This is for the ?60 types, but it looks just like our 2010 S80 V8's instrument panel.
https://www.youtube.com/watch?v=jeb2DRk71Cc
Seems like you can bring up the speed digitally this way.
The digital speedo as such doesn't appear in the 'speedo' window, but in the messages window of the tacho, and it's more part of the trip computer readout - hence is displayed as 'current speed'. This may take some faffing around to find the correct button combo. (Wife and I have had our S80 for 6 years, and we haven't worked out one-tenth of the functions yet....)