Many of them have a "dashboard" mode, where they give distance travelled, speed, Vmax Vavg
This is from Garmin :
TomToms usually display speed on the navigation screen but can have Tripmaster installed to display more trip info on a seperate screen.
A smartphone with GPS is also an option.
It likely won't be cheap, but Garmin has / is about to release a GPS with OBDII functionality.