Your speedometer may be a little worse than most, but they almost all read the wrong speeds, and it sometimes even varies from winter to summer as to how bad it might be.
The solutions might be to get a bicycle speedometer and install the sensor on the front fork, or get an expensive GPS speedometer(if you can find one that fits in the hole in the dash), or get a tachometer and learn how to relate that to your speed.
None of us really can depend on our speedometers with Bullets.