GPS inaccurate


 I used my new watch for the first time today to record  a 5km run. The gps was awful. It recorded me doing 6.64km. The device is up to date and regularly synced to the app. I allowed it to connect fully before setting off and am using all satellites options. 

i went for a follow up test walk of 2km which it recorded as 3.4km. 

There are no tall buildings. Largely open space. Sunny with some cloud patches. 

anybody else have terrible gps? Thinking of getting a refund… a shame because all the reviews highly rated the gps.