Thanks brother
Advice Noted ...
The Speedometer and Navi Screen Speed does not always match but it is obvious .. The GPS has variety of Error sources which are inherited i.e.
-The built in clock of the receiver is not as accurate as atomic clock of satellite
- Sometime Satellite Visibility is poor due to obstacles
- Unwanted Signal reflections mess up the calculations
Well the list is long , Point being it is never accurate , The tacho in your car takes reading directly from the tacho transducer which should serve as a reference Speed .... and YES changing tires to bigger tires, you compromise on Speedo Calibration. Although the Difference is not much and neither annoying