Conclusion of thumb is that the speed of the vehicle speedometer than the GPS is more accurate value.
Regulations to keep the car manufacturing, auto safety standards, Article 110, or at a speed of 40km an hour when the speedometer instructions juhaenghal error '0 ≤ Instruction error ≤ (Actual speed/10) +2' is defined as can.
Thus, the normal car's speedometer, both higher than the actual driving speed is the speed instructions.
GPS speed of about 1-3 seconds before it displays the correct speed, so much more accurate than the speed of the vehicle.
Easily tell you the actual speed of the vehicle's speedometer up to 10% faster than the set has been released.