Hi raincloud,
It has nothing to do with the Doppler effect. The speed shown on you car speedo is always below the actual speed so that you cannot sue a car manufacturer when your "speedo showed I was doing 30!" and a cop gave you a ticket and said you were doing 35mph.
In the UK the difference is (minimum) +10% + 2mph. E.g.: If you are really travelling at 30mph, your speedo will show 35mph.
As long as your are traveeling at a steady speed, not accelerating or decelerating, the speed shown on your GPS is correct.
