Its quite common for a GPS device to display a speed that is less than the speedometer speed. I have used a few GPS units before and the speed shown on the device is usually plus minus 10km/h less than the speed shown on my speedometer.

However, most people advices that the speed shown on the GPS should be trusted more than the speed shown on the car's speedometer.

"GPS is more accurate, speedos are set 5% to 10% high since manufacturers would get sued if they said people were doing the limit when they are in fact speeding. Speedo is effected by tyre wear and pressure, plus the mechanical/electronic accuracy."

"GPS will jump around though if poor reception, and will be confused by twisty roads or changing speeds. It needs to average out its reading."

"On straight highway at a stable speed the GPS will read almost perfect."

GPS, Car speedo, or roadside speed indicator. Which do I believe?

GPS Speed accuracy