The dead reckoning performance of a system shows how well the system tracks speed and position when there is no GPS data at all. This gives a good indication of the performance through tunnels and under bridges. The main limiting factor on the dead reckoning performance for speed measurement is the accuracy of the pitch gyro. Drift on the gyro means that changes in gradient can be interpreted as acceleration changes which result in false changes to the speed.
The following data shows the performance of the system with regards to speed and position accuracy when there is no GPS data present.