I just bought a new Nokia phone C3-05 with built in GPS so today i tried it out against the inbuilt GPS speed in the RNS 510 ,Both were with in 1 to 2 km per hour at 110kmh.
And comparing both against the car speedo to get a speed of 110 gps i had to sit on 120kph so in future i will use
Dead Reckoning in the RNS 510 secret menu .