How accurate is vehicle speed on a GPS ?

Pretty accurate. Unless you're going thru the mountains it will jump a few mph but for the most part my gps is 99% accurate on speed

 

I've compared GPS speed to speedometer speed and find the same thing as you. The GPS speed is more accurate when approaching construction zones or toll booths than the speedometer, when they have the speed display on radar.

I put a new set of drives on last fall, and noticed about 3 mph difference at 70 mph. Now that they are worn the difference is in the 1.5 to 2 mph range.

 

The Rand McNally 530 I have is always accurate.

 

Yeah, that would explain part of my owner's mortification. Here I thought it was because of the cold weather.

 

Many phone apps to check speed also. I have... speed view....

 

When I drink beer, it's Chimay.

 

It depends. But in truth if you are within 3 mph, you are safe.

The gps engine that many many units have receive a signal from at least three satellites and processes the information to output it in one second intervals. For the most part there is an error with gps units, many times a few mph. If the gps system is using a .1 or .02 timing interval, the accuracy is greatly improved.