How accurate is my SatNav's speedometer?

I’d venture that it depends how often your satnav (GPS?) updates and how fine the resolution on it is.

Basically it’ll compare your last position with your current one, determine the distance and divide it by the interval time (v = d/t). I suppose if it were more sophisticated it would keep a small history and provide an average across a window of time. Some satnav/GPS systems have lower resolution or accuracy (location is +/- a certain number of meters) so that can also affect your calculated velocity.

Personally I’d stick with your vehicle’s speedometer – it will likely be much more accurate and probably easier to read while driving. But if you’re saying you think it’s losing accuracy at higher speeds, bite the bullet and get it checked out by a mechanic or something.

i disagree.

even inexpensive GPS boxes are accurate to around 1/10 of 1 km/h. the speedometer in your car, on the other hand, is deliberately manufactured to make it look like you’re going faster than you are. car companies do this so that you won’t get a speeding ticket and then try to sue them for giving you a speedometer that registered too low.

trust the GPS unit and get your car looked at.

I agree with sam. thanks to a lack of detail in the personal information section of this site I can relate the following story on why. I drive an 06 honda accord coupe 4 cylinder, the american version (which is much less powerful than euro accords, at least in 4cyl) and the manufacturer’s reported top speed is 128mph. I got curious one night, and set my Garmin on the dash (its a lower-end model) and went out to a strip of seldomly-patrolled highway. the GPS reported 128, and the speedo said ~131. Fortunately, I don’t know what a cop’s radar gun would have said, but probably something along the general idea of “turn your license over to the state of massachusetts”

I’ve actually had radar guns report my car traveling faster than my speedometer registered. Specifically 2 mph faster. I have Mazda Protege. It really comes down to calibration. If you changed the tires at all from standard it will change your speedometer readings.

The GPS units take an average of your travel over several seconds, using data that is accurate to a few meters, on the other hand your speedometer is made specifically to determine instantaneous speed from direct calculation. Also, if car companies do change the shown speeds then they must all change them exactly the same because every car I’ve seen traveling on cruise control at 80 was traveling at almost exactly the same speed. If you want to claim conspiracy you have to explain why every car company has the exact same standard for doing this. Also, performance nuts that race their cars would be extremely angry at car companies if they did this (I’ve never heard anyone complain about this).