speedo vs sat-nav speed

[MikeMc]

Hi,

Is it just me or does your car speedo always disagree with the speed indicated by sat nav. For example on the motorway my speedo indicated 80mph whereas my TomTom reported me doing 71mph. I am inclinded to think the sat-nav is the acurate measurement but when I go past a speed camera I use the vehicles speedo just to be sure. Does anyone know which is more likely to be correct?

Mike

 

[WatsonJohnG]

I have the Aldi speed camera indicator and that also shows a lower speed than the car speedo

I would trust the sat nav more than the car speedo

If I tow at an indicated 60mph on the car speedo I am overtaken by lorries that are governed whereas if I go off the speed camera reading I am not overtaken except by foreign registered ones

If you know what I mean Patrick!!

 

[BeemerMal]

I think this is the same for all SAT NAV and Camera Detector units that rely on GPS.

Vehicle manufacturers speedos usually record up to 10% less than the actual speed. I was told that they do this in order to stay on the "right" side regarding recording actual speeds.

I believe this is also accepted by most police forces where speed checks often allow for the variance between devises.

30mph = 33, 60mph = 66, 70mph = 77. Therefore the police are not likely? to prosecute anyone unless they have exceeded the higher figure.

However, saying all that I personally prefer to use my car speedo reading....just in case and to be on the safe side!

 

[Lutz]

In order to avoid the possible negative consequences of a low reading, car speedos are purposely set to show conservative, i.e. higher, values. Also, the accuracy of the speedometer depends on the rolling diamter of the tyres. Low tyre pressure will therefore result in a higher reading, as may a change to a different tyre size or make. A quality sat nav, which is independent of the number of revolutions of the wheels over a given distance, should therefore always be the more accurate of the two.

 

[Robert_374242026]

My TOM TOM 700 always reads 4 mph slower than our car speed, when towing i always have to nudge up the speed .When in rader places i drop to the car speed.

 

[RogerL]

Sat nav is almost accurate - speedometers are required to read fast, ie between 0% and 10%+2mph - to facilitate production and allow for tyre wear all speedos under-read.

Mike - at 80, for a satnav 71, your car speedo is at the upper limit of legality.

 

[Murdan]

I get 2 mph between the sat nav and the car speedo the sat nav being the lower speed.

 

[Andrea_374349948]

I get 2 mph between the sat nav and the car speedo the sat nav being the lower speed.

I previously owned a vauxhall Astra & Zafira for which reported about 10% higher than sat nav. Now have Peugeot 407 and reports the same as sat nav. Sat nav is more accurate but I would not use this as defence if got stopped by the police for speeding.

 

[martin.]

We've got a Tom Tom one Europe and I use it in my van for work, in a Transit 70tomtom=76speedo, in a sprinter 70tomtom=75, renault master 70 tomtom=73, worryingly the vw read 9 mph over the tomtom.

 

[ProfJohnL]

As Lutz has pointed out EU vehicles speedos will always be a little optimistic, showing a slightly higher speed than the vehicle is actually travelling.

The degree of error is not necessarily the same for vehicles of the same model, so it is misleading to assume that your vehicle will have the same error as your friend with the same car etc.

As For Sat Navs, they have the potential to be far more accurate than the vehicles speedo, but just because it is digital does not make it fool proof. As with any digital instrument there are conversion errors, and in general you should not trust the accuracy of the least significant digit.

It is wise to err on the side of caution, so always allow at least 1 count of the LSD as headroom. So for example use 69 rather than 70. and similarly for other speeds.

It is also worth remembering that the accuracy of the SAT NAV can diminish with slower speeds giving a greater error

Please remember the UK national derestricted speed for solo cars is 70, - not 71.

 

[ShadySadie]

"Please remember the UK national derestricted speed for solo cars is 70, - not 71."

How loud and how long should we laugh!

It's also 60 mph for caravan outfits, maybe there is an amnesty for Easter caravanners!

 

[Chrisbee]

Whilst I tend to agree that sat nav is the more accurate I wouldn't rely on it for speed indication purposes. My new Garmin is slow to follow the speedo indication when I move off and sometimes when I accelerate gently the speedo and rev counter rise but the sat nav doesn't although it will catch up after a few seconds. Also, when I stop it takes a few seconds for the speed to show zero, i.e. it shows me moving when I am stationary.

Chrisbee

 

[ShadySadie]

Crisbee.

Your sat nav may not be well positioned in the car and do you regularly download Garmin software upgrades.

Our Satnavs, Garmin and Tom Tom speed moves with the speedo.

 

[Dave_374330759]

You should be aware that there are errors within the sat nav system that will not show up in a basic car unit, these are related to the fact that the satellites themselves are moving and various other atmospheric effects. So a basic handheld GPS reciever will show some movement even when the receiver is stationary. So to eliminate these effect there is a degree if damping applied so the instananeous reading may be a couple of seconds behind the actual - so although thoeretically the satnav should be more accurate than the speedo, don't rely on it when you are going past speed cameras (you don't know what the've been set to)

 

[rob_jax]

The difference that Chrisbee observed is due to the different way that a car speedometer (and rev counter) measures speed compared to a Sat-Nav. Updating the Sat-Nav software or its position in the car won't have any real effect.

A car speedometer uses a sensor on the gearbox output shaft to measure the rotational speed - and this is then converted into a speed display - and is almost instantaneous in it's response to changes.

A Sat-Nav displays speed by continuously calculating distance / time - both of which it knows with a high degree of precision - so the displayed speed is (should be) highly accurate - but it needs a second or so for the car to travel a "significant" distance. If this was displayed to the driver it would appear to be "instantaneous" too.

But the sat-nav manufacturer chooses to average a few speed readings and display the speed at a slower interval - every few seconds - so then you do notice the lag between displayed and actual.

If you start off from rest and then reach 10 kph in one second - the sat nav will display the average speed for that time period - which will be 5 kph - the lag is due to the measurement and display process - it smooths out rapid changes.

If you drive at a constant speed (or effectively constant), and if the car manufacturer used a good quality speedo and sensor, and if your car is running on correctly inflated, nearly new tyres as specified by the car manufacturer - then the observed difference can be very small:

Last week commuting through Belgium, I set my car speed limiter to 120 kph and the Sat-Nav showed a "true" speed of 118 kph - a difference of rather less than 1% - but as the tyres wear down, then this difference will increase.

But different car manufacturers will choose to use different "speed offsets" in their car computer that converts gearbox output rotation into displayed speed.

Robert

 

[ShadySadie]

Even I know that satellites are in a Geo Stationary orbit so they stay in the same place in comparison to the ground!

Speed rating from Satnav is safer than speedo.

They rely on satnav to position and park Airbus A380 a car should not be a problem in comparison to inaccurate speedometers.

 

[Dave_374330759]

Even I know that satellites are in a Geo Stationary orbit so they stay in the same place in comparison to the ground!

Speed rating from Satnav is safer than speedo.

They rely on satnav to position and park Airbus A380 a car should not be a problem in comparison to inaccurate speedometers.

Communication/TV satellites are geostationary, Navigation satellites are moving all the time at high speed, that's why it takes some time for a GPS to actually register it's position. The receiver has to know where to look for them, so if the unit has been switched off for several days or its location has been moved significantly (50-100miles)since it was last used it doesn't know which satellites are in view or where they are, it has to search until it receives a satellite message to tell it where they all are - that why if you've got a Tom-Tom you can connect to the internet and update the positional (ephemeris) information to speed up the aquistion time

If you use a handheld navigation GPS rather than an in-car type you can see which satellites are in view and where they are located in relation to the receiver. Sometimes they can be in positions which are not ideal for position fixing or there may not be enough satellites actually in view of the receiver.

 

[Dave_374330759]

Even I know that satellites are in a Geo Stationary orbit so they stay in the same place in comparison to the ground!

Speed rating from Satnav is safer than speedo.

They rely on satnav to position and park Airbus A380 a car should not be a problem in comparison to inaccurate speedometers.

They may use satellites to park Airbuses and even navigate aircraft but they don't use cheap and cheerful kit like you get from TomTom, Garmin etc. nor do they use the same satellite information. The equipment they use is specially licenced and has a much higher accuracy (mm rather than metres) than the c/a signal (coarse aquisition signal) that satnavs use.

 

[Dave_374330759]

The GPS system works by using a network of 24 NAVSTAR satellites orbiting the earth. NAVSTAR stands for Navigation System using Timing and Ranging. Functioning 24/7, at a height of 10,900 - 12,625 miles above the earths surface. The satellites weighing 2 tons, are 18.5 feet long, transmit on two frequencies: 1575.42Mhz for civilians and 1227.60MHz for military services. NAVSTAR satellites orbit the earth in just 12 hours

 

[ShadySadie]

We thought modern automotive Satnavs were WASS (Wide Area Augmentation System. It's a system of geostationary satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy than currently available with the standard Global Positioning Systems) /EGNOS(European Geostationary Navigation Overlay Service) enabled to provide todays better accuracy!

Our pilot sons jets use more advanced encrypted system, but his automotive Garmin still manages to give the same speed readings!

We also understand that the cross channel ferry satnav readouts come from the boats advanced shipping system, our satnav gives the same speed read out and same positioning as weel as we can see.

 

[rayc]

The speed measuring device that really matters is the one used for detecting you were over the limit and prosecuting you. There is no arguement, it is readily accepted by the courts even when some are used at ranges in excess of 700m.

 

[ProfJohnL]

"Please remember the UK national derestricted speed for solo cars is 70, - not 71."

How loud and how long should we laugh!

It's also 60 mph for caravan outfits, maybe there is an amnesty for Easter caravanners!

Hi Sadie,

My comment regards the national derestricted speed was qualified as for a solo car and I mentioned it because a previous respondent had used 71 mph in their answer.

As for the actual limit, there is no law that automatically allows speed (sorry safety) cameras to give any tolerance for exceeding the speed limit. In fact it is down to the decision of the prosecuting authority to decide to proceed with a prosecution or not. They may apply some latitude but it is at their discretion and it may be based on the recommendation of the areas Chief Constable. By the same token they can decide on a zero tolerance, thus 71 for a solo car could be prosecutable. It is worth noting that some CPO's have expressed a desire to move towards a zero tolerance for speeding. - and they can without prior notice being given.

 

[drdick&vick]

I have tested my speedo against the following:

Kenwood DNX 7200 with Garmin sat nav

Tom Tom 920

Garmin GPS unit x 2

TomTom 710

I have had one of the Tom Tom's and a GPS unit in the car at the same time as the DNX was running and all three units were reading 2 mph less than the speedo on my 07 reg car.

I will trust mt GPS/Sat Nav each time

 

[DeliDave]

Hi Sadie,

My comment regards the national derestricted speed was qualified as for a solo car and I mentioned it because a previous respondent had used 71 mph in their answer.

As for the actual limit, there is no law that automatically allows speed (sorry safety) cameras to give any tolerance for exceeding the speed limit. In fact it is down to the decision of the prosecuting authority to decide to proceed with a prosecution or not. They may apply some latitude but it is at their discretion and it may be based on the recommendation of the areas Chief Constable. By the same token they can decide on a zero tolerance, thus 71 for a solo car could be prosecutable. It is worth noting that some CPO's have expressed a desire to move towards a zero tolerance for speeding. - and they can without prior notice being given.

spot on John,didnt chief constable in North Wales give Zero tolerance and ticket almost everyone? its a myth about 10% allowance.

 

[ProfJohnL]

Hello Richard,

Whilst Sat-Navs have the potential to be more accurate than the speedo in your car, they can produce erroneous readings, and give a false position and speed indication. It is unwise to assume they are 100% accurate.

By reason of their uncertainty of position (+or- a few Metres)at slow speeds they can have a significant speed indication error. The error diminishes as your speed increases.

Comparing them to your speedo must be done across a range of speeds. You might have a 2mph error at a particular speed, but that error may change its value at different speeds.

because of the mechanical nature of your speedo, it has to convert a rotational input to a deflection. The types of errors that can and do exist can include Conversion factors, linearity errors, zero offsets.

I do use a Sat Nav to set my cruise control when on the motorway, and I set it to a reading of 69mph (solo Car) which should account for errors, and any additional speed build up when going down hill.