What is correct, Speedo or sat nav?

james2012_2

I bought myself a sat nav and set it up in my car.

Now I know why everyone passes me when I am going along the road at 70 MPH.

The speedo shows 70 but the sat nav shows 63.

When the speedo shows 80 the sat nav shows 72.

Surely my speedo can't be as far out as that?

Loanranger

My speedo is two mph slower than actual speed of the car. My sat nav confirmed this. The sat nav was correct.
I have a speed limiter on my car and when set at 30 mph the car is travelling at 27/28mph.

Foggster

Having recently attended a Speed Awareness course and grumbling about the fact I was "only doing 34" I was politely informed that a speedo is set around 3-4 mph slower than the true speed.

A sat nav, I would assume, is using an "average" because it is having to get its reading via a satellite and there has to be some delay surely??

Very good question and I am sure there are more knowledgeable peeps out there who will blow my theory out of the water!! LOL

Lum

On a straight, level road, at a constant speed, with a clear view of the sky, the GPS will be accurate. Any bends, hills or changes in speed and the GPS will be inaccurate. As the above poster states there is some "lag" or delay in the speed reading from a GPS.

The speedo is always inaccurate as when they are made they are not allowed to underread but are allowed to overread by up to 10%. The inaccuracy is pretty much constant, so if you calibrate the speedo in your head using a GPS as described above, ideally using cruise control, then you can then rely on that for the rest of your journey.

Your speedo will, however, overread more as the tread wears away on your tyres, so it's worth checking it against the GPS every week or so if you are going to use this trick to, for example, save time by doing exactly 50 through a SPECS camera network. You should also check the difference immediately after replacing your tyres or you could end up going somewhat faster than you thought.

KillerWatt

james2012 wrote: »

Surely my speedo can't be as far out as that?

Yes it can.

Speedo's are NOT allowed to under-read by law, however they are allowed to over-read by as much as 10%.
From a calibration (and hence, cost) point of view, this suits the manufacturers just fine as it's easy to calibrate a speedo to read one way or another accurately enough, but slightly more involved if you want it bang on the money to the MPH right across the whole range (aka, cop car).

jase1

Foggster wrote: »

Having recently attended a Speed Awareness course and grumbling about the fact I was "only doing 34" I was politely informed that a speedo is set around 3-4 mph slower than the true speed.

They were lying to you -- it's 3-4mph faster!

property.advert

My speedo is around 7% faster than my sat nav and that is over any type of road.

TheSavingGuru

The more accurate the Sat Nav the more accurate the speed...

I.E. the more satellites you have on tomtom the more accurate the speed.....

Speedo is always inaccurate on 99% of cars....

missile

Only 1 mph difference between my Audi Speedo and my TomTom sat nav at 70 mph.

jcb208

missile wrote: »

Only 1 mph difference between my Audi Speedo and my TomTom sat nav at 70 mph.

My vectra is 1mph different at 30 and 2mph at 70

C_Mababejive

In my view,the speedo is more accurate since it is more closely connected to the medium that is to be measured i.e the road although of course we must allow for the margin of error allowed in manufacture.

Anyway the police are interested in your speedo,not your sat nav.