Speed meter

[Mic1.4]

I use PDA GPS navigation and the speed on it is about 5-6mph less then car speed meter.I keep constant speed for about a half a minute.The tyres size is original(165/60R/14).I believe the GPS should be correct but the difference is too big...
Is it possible to be wrong programming of speed meter?

 

[Arnold]

The gps will be correct. Dials can over read but are not allowed to under read at all, so i think manufacturers set them a little bit too high. does your car definitely have the factory standard wheels?

 

[TimmyB]

Saying that the most accurate GPS for cars is only within 3 metres. So no the GPS won't be 100% accurate either, so it probally a combination of the speedo set high and the GPS's 3m accuracy!

 

[Ed]

To detect speed GPS receivers use the Doppler effect, which is the phase shift relating to how fast your moving towards or away from the GPS transmitter. Therefore its very accurate, and isn't affected by the positioning accuracy.

 

[capnmchl]

When I borrowed my friends GPS to go up to the staffordshire meet at the beginning of march, what I though was 70MPH, was actually only 64 or something.

It seems to be exponential though. At lower speeds, the difference isn't so great, but the faster you go, the more out it gets. Like a constant 10% error.

 

[Krazy]

We noticed that on saturday night after borrowing a friends satnav

 

[k.pol]

the faster u go
bigger the gap

i was driving with 130km/h and mio gps with 123km/h ( if i remember right)

 

[Mic1.4]

It turned out that all of you have the same experience.That means the car speedometers are factory adjusted higher...
On my last car ,with the same TomTom navigation I had a deviation as well but 2-3 times less.

 

[capnmchl]

I guess the easiest fix is just to bung another 5mph on your speed and it'll average out.

But I guess bigger wheels will sort it out properly.

 

[Robert]

195/50/15 tyres made my speedo read accuratly with satnav