GPS speed is always more accurate than car speedometer.
GPS is always connected to many satellites at a time and doing commication constantly. If u are going on a flat straight road, normally a car speedometer may have a calibration difference Upto +5%.
It's always the case. GPS will show 4 to 5km/h less speed. That is also one of the reason that at motorways, police let you go without ticket UpTo 130km/h or little less. Normally I set my cruise at 129 128km/h and never got a ticket at this speed.