Speedo v Sat Nav

[Don Madge]

Morning all,

 

I tried out my new Garmin nuvi 1490T in the Timberland and the car (Renault Megan Scenic Dynamique DCI 130) 06 plate.

 

In the van the speedo was about 3 MPH slower than the Sat Nav.

 

In the car when travelling with the cruise control set the CC display gave the same reading as the Sat Nav but the main speedo (digital) was reading 1MPH faster than the Sat Nav.

 

We don't usually drive to the max speed limit but a moment's inattention could get as a speeding ticket.

 

How accurate are the Sat Nav's?

 

Can the cars speedo be adjusted?

 

What are my options please?

 

Safe travelling.

 

Don

 

[Dave Newell]

The sat nav is as accurate as you're going to get Don. The speedo can legally read up to 10% high but must not read lower than your real speed. Sat nav works by calculating your position in three dimensions (although some people will say its only 2D) based on the time taken for a signal to reach it from the satellites. It needs to "see" at least three satellites to work, the more it can "see" and the further apart they are the more accurate it becomes. Typically sat navs check location once per second and calculate speed by simply measuring how far it has moved in one second (or whatever its frequency of position checking is). This means that the speed display is showing you how fast you were travelling one second ago but this is close enough to be deemed accurate unless you're flying a Tornado. The speedo on the other hand is driven by your wheels, either via a pulse generator at the gearbox output or by the wheel speed sensors fitted for the ABS system. Systems driven from the ABS sensors often take an averaged speed from the ABS ECU to drive the speedo (during cornering all four wheels will be turning at different speeds). As your tyres wear the rolling radius decreases making this method of speed measurement not so accurate.

 

Typical discrepancy is 6% over on the speedo in my experience.

 

D.

[enodreven]

Hi, Don

 

I have 3 differnt sat navs and all of them read slower than the speedo on both my car and motorhome,

 

Which has always given me a degree of comfort when passing speed cameras, in that if following my speedo reading I would actually be under the given speed limit for the camera and not get a ticket.

[flicka]

Hi Don

This was discussed briefly on a previous thread regarding MPG.

In my case the Van speedo is reading faster & distance travelled, than the Sat Nav by 5%

When using the Sat Nav in the Car the difference is 3%.

As Dave said the Sat Nav is as accurate as you can get, with the allowable tolerance for vehicle speedo's.

Same applies when calculating MPG values actual against trip MPG on the speedo.

[Brian Kirby]

The simplest answer, I think, Don, is to rely on the speedometer.  It is nice and big, legally required, and easy to read.  It is liable, as Dave says, to be conservative, so has an in-built safety margin.  Trying to match sat nav speed to speed limit is liable, IMO, to result in far too much attention being paid to the sat nav, and insufficient to the road!  Besides, you'd never get anywhere significantly earlier by exploiting the difference between the two - and you be flirting with every Gatso along the way!  Too much like hard work, with a reward hardly worth the effort.

[Don Madge]

Brian Kirby - 2010-07-25 1:31 PM

 

The simplest answer, I think, Don, is to rely on the speedometer.  It is nice and big, legally required, and easy to read.  It is liable, as Dave says, to be conservative, so has an in-built safety margin.  Trying to match sat nav speed to speed limit is liable, IMO, to result in far too much attention being paid to the sat nav, and insufficient to the road!  Besides, you'd never get anywhere significantly earlier by exploiting the difference between the two - and you be flirting with every Gatso along the way!  Too much like hard work, with a reward hardly worth the effort.

 

Thanks everybody for the replies.

 

The original post have read can the vans speedo be adjusted as it's reading about 3MPH slow compared with the sat nav. If the sat nav is reasonable accurate I will have to be very careful with the van speedo reading slow.

 

Brian,

 

I think you missed the point of the post :-D I was not looking or planning "get anywhere significantly earlier" as you put it but just wanted to put my mind at rest.

 

As for "flirting with every Gatso along the way" 8-) Having been driving now for 58 years still with a clean licence (I did get done for speeding in Turkey last year but I don't count that) I can assure you that's the last thing I do.

 

Safe travelling.

 

Don

[Dave Newell]

You may have the wrong sized wheels on your van then Don and if your speedo is reading slower than true speed then it is not legal as well as leading you potentially into speeding fines and points. Its possible that your van should have 15"wheels but these were replaced with alloys of 16" diameter which could possibly have been changed by the converter. The only possible ways to alter the speedo reading would be to swap the tyre/wheel size or possibly fit a different speedo pinion drive.

 

D.

[Brian Kirby]

Don Madge - 2010-07-25 2:00 PM ............... Brian, I think you missed the point of the post :-D I was not looking or planning "get anywhere significantly earlier" as you put it but just wanted to put my mind at rest. As for "flirting with every Gatso along the way" 8-) Having been driving now for 58 years still with a clean licence (I did get done for speeding in Turkey last year but I don't count that) I can assure you that's the last thing I do. Safe travelling. Don

Yep, think I did, Don!  I thought you were planning on driving to the sat nav and not the speedo.  Me, question your prowess?  The idea!  :-)

I tripped a Gatso in France last autumn.  Have heard nothing to date, so we'll either get pulled as soon as we roll off the shuttle, or there was no film inside.  Bummer!  :-(

I also misunderstood what was slow twixt van and sat nav.  Slow, your van speedo should not be.  Dave's theory is good.  On our transit it is possible to "tell" the speedo what size tyres are fitted.  I can't, but a dealer can do this with a laptop plugged into the electronics.  I wonder if your Fiat may have a similar facility?

[mirage]

Our sat nav has read 0-3MPH below three other vehicles speedo`s that I have compared it too. I would suggest any speedo reading under a sat nav is highly suspect particularily if trying to avoid speeding fines. Its also interesting to compare the vehicles trip distance over a long continental journey to that of the sat-nav trip distance.

[Madmaggott]

We tend to go by the sat nav speed when abroad as the km markings on our speedo are so faint as to be almost unreadable whereas a only quick glance at the sat nav is required.

In the UK we use the speedo.

 

[Porky]

Madmaggott - 2010-07-25 8:43 PMWe tend to go by the sat nav speed when abroad as the km markings on our speedo are so faint as to be almost unreadable whereas a only quick glance at the sat nav is required.

In the UK we use the speedo.

I agree. We spent 4 months in Spain and France and I used the TT. Also one useful feature is that the speed shows up red if you are exceeding the limit. Our speedo is a couple of Kms slower than TT.

[tonyg3nwl]

Hi Our mio sat nav has been set up for speed cameras and automatically shouts "you are over the speed limit" when it thinks you are in a limited area if you are indeed exceeding the limit. Trouble is where the area has altered in recent years, it doesnt know the new area limits or start /end positions. Locally, there is a 50 limit start point which has shifted about 1/2 mile, which was previously 60, so no warning given until potentially too late, in other cases it shouts over the limit whereas the actual restriction has gone up from 30 to 40, so you need to be alert to watch for yourself.

 

I find it useful, if misleading at times.. As far as camera positions is concerned, it doesnt know where mobile traps are obviously, and I understand that in some countries, the warning for fixed cameras should be disabled. I cant see the logic of that, surely it is better to get a warning and reduce speed appropriatley.

 

Difference between Sat nav and speedo at speedo indication of 55, sat nav says 52.5 and at speedo 60 satnav says 57. I tend to trust sat nav.

 

tonyg3nwl

[Guest JudgeMental]

"you are over the speed limit" being shouted at me repeatedly would drive me insane!lol

 

No the Garmin is much nicer, you get a different tone "Bong" that alerts you that you are near a camera and going to fast, and when you look at unit it has a red bit at top ..you slow down and it goes back to the normal green...

 

you need to upgrade the camera data, my garmin is updated monthly.

[starvin marvin]

Don, Get your speedo calibrated to the correct speed reading, not expensive and you will be pleasently surprised how much faster you can drive and still be legal! I use Speedograph in Nottingham. Then forget the sat nav and as Brian has said read just the speedo.

[Clive]

Or a revolving number plate and a mask!

 

Your van speedo reading slow is very sus!

 

C.

 

 

[Don Madge]

After reading all the replies my my original post I realised I was totally confused and decided to take the van out and do a longer road test.

 

Fortunately where we live in East Yorks we have a reasonably flat 8 mile length of dual carriageway. I did approximately 500 metres at each speed.

 

This was the result

 

Van 40 Sat Nav 37

 

Van 50 Sat Nav 47

 

Van 60 Sat Nav 56

 

Van 70 Sat Nav 65

 

I did the check for peace of mind and not so I could drive any faster.

 

In fact when I was doing 70 in the van I almost "evacuated my bowels" as I don't normally drive at anywhere near that speed.

 

So there you have it I'm a happy man again until the next problem pops up.

 

Safe travelling.

 

Don

[starvin marvin]

As you can see, as the speed increases the "gap" gets bigger. Tried mind at 130 kph on the auto route back to Calais. That pinned my ears back! loved the 2.8 jtd it could really shift, ours was called the Silver Bullet.

[tonyishuk]

Don Madge - 2010-07-26 12:52 PM

 

 

Van 40 Sat Nav 37

 

Van 50 Sat Nav 47

 

Van 60 Sat Nav 56

 

Van 70 Sat Nav 65

 

Don

 

Van 80 Sat Nav 75 (Only on the Peage !!)

 

agrees with my TT XL readings.

 

Only to add the Fiat White Van Man Warning seems a little variable, it is set @ 70 MPH but seems to operate anything between 65 and 75 mph. A useful beep in some circumstances !

 

(Newer Fiats have a speed warning that can be set by the driver to warn when over a preset speed)

 

Rgds

[Clive]

"when I was doing 70 in the van I almost "evacuated my bowels"

Your getting Old Don, Janet does that on her Monkey bike!

 

C.

 

[Guest Tracker]

Whilst the sat nav speed indication is accurate for constant speed use do bear in mind that it is reactive and not accurate when accellerating or slowing down.

 

I've never had a car or a van yet where the sat nav speed agrees with the speedo when cruising.

[tonyg3nwl]

JudgeMental - 2010-07-26 9:43 AM

 

"you are over the speed limit" being shouted at me repeatedly would drive me insane"

 

yes, but better than having the boys in blue shouting it

 

" and when you look at unit it has a red bit at top ..you slow down and it goes back to the normal green..."

 

Thats fine except I am colour blind, and cant tell red from green..... and before you ask, the red one is at the top on traffic lights.

 

 

"you need to upgrade the camera data, my garmin is updated monthly."

 

This can be done on line but relies on somone sending in the updated positions and changes in the first place.

 

tonyg3nwl

 

 

sorry didnt get the quotes in the right places tony

[bigal55]

JudgeMental - 2010-07-26 9:43 AM

 

"you are over the speed limit" being shouted at me repeatedly would drive me insane!lol

 

No the Garmin is much nicer, you get a different tone "Bong" that alerts you that you are near a camera and going to fast, and when you look at unit it has a red bit at top ..you slow down and it goes back to the normal green...

 

same as the tomtom then, it does exactly the same, i set mine to moo because i,m sad

[sshortcircuit]

I stand corrected, but I understand satnav measures your horizontal speed and when you are on an incline your surface speed can be higher than satnav indicates and the surface speed is measured by the Police.

[Dave Newell]

sshortcircuit - 2010-07-29 7:07 AM

 

I stand corrected, but I understand satnav measures your horizontal speed and when you are on an incline your surface speed can be higher than satnav indicates and the surface speed is measured by the Police.

 

Not necessarily correct, it will depend on the software in use in the sat nav unit. All GPS systems work by calculating a 3 dimensional location in space, often a a rate of once per second. Speed is simply calculated by measuring the distance moved during that second and should therefore be the speed in a direct line between the two points in space. A simple test would be to take a GPS unit into a lift and assuming a signal could be acchieved see if it gives a speed reading during ascent/descent of the lift.

 

D.

[Keithl]

Dave Newell - 2010-07-29 1:56 PM

A simple test would be to take a GPS unit into a lift and assuming a signal could be achieved see if it gives a speed reading during ascent/descent of the lift.

 

D.

 

Or jump off a cliff with it! LOL.

 

Edit. Sorry Dave, couldn't resist that one. K.