how accurate are gps's

[trax de max]

i was doing some speed runs last week and according to my gps i was doing 25.6 mph.I bought it from local market place with out instructions but managed to download some.
But the speed has to be wrong but don't know how set it up its a magelan map 330 gps. How accurate are gps's

I was expecting speeds of very close to 40Mph 45 tops.

 

[BamaFan]

GPS is the closest you will get to a accurate reading on your speed. Speed odometer's are not as accurate as a gps.

 

[trax de max]

i must have some settings wrong on this, i have noticed if i stand still the distance mileage goes up.
this is a run in wet grass, i took the speed on road in dry.
http://www.youtube.com/watch?feature=player_profilepage&v=nKU0V86pkJM

since i took the readings i changed the spring on the slipper incase the spur was just slipping too much. Haven't managed to get any readings yet though.

 

[beason]

gps is about as precise as you can get. even police radar isnt as precise as the GPS system.

you can take it in your 1:1 car and see if its close, but i would bank on the gps.

these little cars look like they are going way faster then they really are.

 

[Rolex]

I've tested mine in the car while verifying it with the car's GPS as well as the Cobra radar detector that shows the cars speed. All 3 read the same, but the car's speedometer was 1 1/2 MPH faster. No doubt due to some tire wear.