how reliable is the GPS Speed on windows phone 7 -

i developing windows phone 7 app , want tell user how fast going. simple like...

current speed: 20 km/h max speed: 33 km/h average speed: 25 km/h

i seem have 2 options: 1) use accelerometer 2) use gps

i'm wondering if people have suggestions on method use.

i prefer use gps (as simpler extract), know gps position not accurate various reasons, how accurate speed?

speed , direction tend more accurate absolute position - because speed , direction not calculated tracking successive position measurements (as might expect). rather, these values estimated using doppler shift of satellite signals, technical reasons less affected factors tend distort absolute position.

by contrast, accelerometer won't useful measuring speed, speed , acceleration 2 different things.