[neilep (OP)]

Dearest Science Peeps,


As a sheepy I of course luff to go for pleasant drives in my motormobile up and down the country lanes and then full blast on the motorway (for the sake of US peeps, 'Motorway ="Highway" innit....sheesh !!)  sometimes I exceed the mind boggling heights of 23 Mph !!

As ewe can see from the true-non-doctored-bona-fide photo, there I am getting ready to go for a drive !!

Anyway, I downloaded an app on my phone which acts as a Sat Nav and also includes a speedomoeter but there is a difference between the speed indicated on the app and the speedometer in my motor !....the one in my motormobile always shows about 2mph faster than the one on my app ! Now i could try it in another motormobiler but, well, I cant be bothered !

so, are the car manufacturers intentionally indicating that we drive faster than we really are or is my phone app inaccurate ?


whajafink ?

hugs and shmishes


mwah mwah mwah !!

Neil
xxxxxx


Driving along, singing a song
Wondering which speed is wrong

[Bored chemist]

according to my sat nav, my table is quite often travelling at a few miles per hour.

[neilep (OP)]

according to my sat nav, my table is quite often travelling at a few miles per hour.

is this why ?

[Bored chemist]

Not that I know of...

[alancalverd]

The specification for vehicle speedometers does not allow them to under-read, but they can overread by 10% + 4 kph.

On a straight road at reasonably constant speed, the satnav calculation, which generally updates every second, can be considered precise. Satnav differential location is better than +/- 1m and timing is correct to umpteen decimal places, so at 110 kph the probable error is about 1.5%.   

[syhprum]

In recent years car speedometers have become considerably more accurate but they are still influenced by tire inflation pressure and the desire of the car makers not wishing to sued by people who trip speed cameras thinking that they are within the limit.
GPS readings are influenced by altitude to a small extent but will generally more accurate than speedometer readings.


   

[Bored chemist]

It depends (doesn't it always?).
GPS knows where you are to about 10 metres.
If you cover a lot more than 10 metres in a second or so then it will give a reasonable indication of speed.(And the average speed over a longer time will be even better).
But, at low speeds- like my table- it's hopeless, because it tries to update too often.

Speedometers are, as has been pointed out, designed to be cautious.

Incidentally, while GPS isn't very good at determining altitude, the height doesn't affect the accuracy much.
It wouldn't be much use for targeting missiles if it was bothered by altitude.

[Colin2B]

For me the accuracy depends on the receiver. I have various kit which uses GPS from a very accurate plotter which uses 2 receivers in case one fails, down to a small exercise wrist monitor.
The wrist monitor is much more sensitive to dropouts due to trees and buildings, so although accurate over a whole run the log can show short busts of high speed catch-up.
I did notice a similar effect with a car system so I checked the speedo against a run at constant speed. So, as I sit back in my sheepskin covered seats, holding my warm, soft, sheepskin covered steering wheel I know that at 30 mph the speedo read high by one needle width and at 60 by 2.
But it is oh so comfortable!