Well, this is not true that speedo meter of cultus and mehran show a higher speed its mean at zero speed their meter should show 10 or 15 speed?????
GPS in mobile phone will never show the exact speed i have also tested it will show speed 5km less.
There is difference of smoothness not meter as comapared to khayber...
So you saying GPS shows 5 km less. you are absolutely right.
Your GPS tracks your location with an accuracy of couple of meters. So as long as its internal clock is accurate, it will give you the most accurate reading, even at relatively low speeds. I have a handheld GPS, and it even measures my speed when I'm walking.
Car speedometers use a sensor on the driveshaft to measure your speed as a function of wheel revolution. Because the tires are made out of rubber, their diameter varies with speed, load, and temperature, even when inflated to their recommended pressure. You'll never get an accurate reading from a car speedometer.