speeding which is more reliable

[v551dln]

I am a new convert to satnav using a tomtom go to get out and about, and I have noticed that it tracks your speed as you go along.

My question is this, on my speedo the reading could be for example 30 mph but on the tomton it is shown as only about 23 mph, this difference is noticable at whatever speed my vehicle is doing with the speedo always reading higher than the speed reported by the tomtom. Which do you think is more accurate and which would be recorded by speed cameras or radar guns etc.

Finally how do I go about getting my speedo calibrated to check its accuracy. Thanks

[Mrs_Pegasus]

Sat Nav uses GPS i.e satellite and there is a delay in the information being beamed. I would say your speedo is correct and your sat nav is just slow at catching up.

If its possible to find a stretch of road where you could maintain the same speed for a period of time you might be able to to prove/disprove this theory. My satnav is always a couple of seconds behind with the speed reading.

[gammaray]

If the camera flashes you're going too fast!

[kramer81]

I thought sat navs were more accurate, though what Mrs Peg is saying may be true. speedos can be 10% over the actual speed.

[NORMANTHEDOG]

your sat nav is precise in its speed.speedos are not allways true,thats a fact.

[THECORBYLOON]

This was given in a House of Lords Written Answer on 12.3.01

�Lord Whitty (Parliamentary Under-Secretary, Department of the Environment, Transport and the Regions) Hansard source
The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.
A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions.�

So, using the speed shown by the satnav-

23 mph converts to 37.01 k/h.

Adding 5% gives 38.86 k/h

Add 10 k/h gives 48.86 k/h which is 30.37 mph

I hope that helps

[rojash]

"the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. "

Seems to be an error here. If you followed these directions, then in a car doing 100 Km/H it would be illegal for the speedo to show more than 14 km/h

[AllYourBase]

I read somewhere that although satnav is a more accurate measure of your speed, it loses accuracy when you are going up/down steep hills.

[THECORBYLOON]

rojash I assumed the 10% was in addition to the true speed and it does go on to say
"The requirements are also that the indicated speed must never be less than the true speed. "

[Toureman]

I know NORMAN is right and thats a fact!!

You cannot calibrate your speedo.

[wduk2005]

i would say that the sat nav is wrong, my tom tom does exactly the same, have you noticed that if you stay at an exact speed but go around a tightish bend for the speed you are going the sat nav indicated speed will go down,

[DirtyHabit]

Some electronic speedo's can be calibrated. My speedo is marked "Certified Calibration" and when tested by VOSA I was told that it was the most accurate they had ever tested.

[DirtyHabit]

In addition to my first answer. I tested my Tom Tom against my certified speedo and found the Tom Tom was just as accurate and very quick to adjust to various speeds.

[v551dln]

thank you everyone for taking the time to answer, I thnk that after everything is considered I am probably slower than I thought and will use both as a guide.

cheers