After renting many different vehicles, I'd venture a guess that the Speedo/Odo is off 80% of the time. The most radical is in the Kia Spectra: Actual speed = 60, displayed speed = 65. Toyotas are also a off the mark in this direction, pretty consistently -- especially the Camry. The domestic brands tend to be close to the mark -- Ford and GM tend to be pretty accurate. I haven't come across a vehicle that is faster than it's indicating.
My theory is that the tolerance given to manufacturers to allow a higher reading: makes people think they're going faster, and they slow down. In the meantime, the miles stack-up and the warranty expires sooner
My xB is reading 10% low on the odometer but I have been adjusting for it all along - it sort of extends the warrenty by 10% which I will not complain about. The Speedometer reads higher than actual of course but the needle doesn't even reach the marks on the dial so I don't look at it much except when reaching redline on the tach. It is nice to see the TomTom, Garmain, ScangaugeII and roadside radar all agree on the speed however.