GPS is actually not a true indication of speed and depends on the terrain.

China
So you are saying that your Boombox indicated speed and speedo both show 113 kmh and the iPhone TomTom app shows 110kmh?

Have you compared the Boombox/Speedo indicated speeds with an actual aftermarket GPS....not the TomTom downloaded app on your iPhone?

The iPhone does not display direct received GPS data and relies on a data connection from either wifi or a Carrier to display the location/speed data that is calculated by triangulation from nearby Carrier base stations where fixed GPS receivers are located.

I will do a test this weekend to see if I get similar results.

I stand corrected China and was not aware that the more recent phones use GLONASS plus A-GPS and not A-GPS only, which is what I was referring to - thanks for the clarification.

Re the Boombox GPS speed and the speedo speed being linked, do you know if anyone has disconnected the speedo signal from the speed sensor as a quick way to confirm this?

Cheers

 

Double post GRR