# Speed v Sat Nav



## Don_Madge (May 1, 2005)

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


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## cabby (May 14, 2005)

On a previous thread it was thought that the sat nav was more accurate. the speedo can be + or - 3 mph at 60 mph.
hiope this reassures you.

cabby


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## Hezbez (Feb 7, 2009)

I've wondered the same thing.

Both my car and motorhome speedos read about 4mph less than the GPS.

I tend to believe the GPS reading. But then again, if I'm wrong :?


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## paulmold (Apr 2, 2009)

I am under the impression that a speedo cannot under-read ie. it cannot read 50 if actual speed is 53. I once had a car which was subject to a recall for that very reason, the manufacturer would have been taken to court for all the speeding fines it may have caused. In my case (a Toyota Avensis) the speedo over-states by 10% compared to my sat nav. When my speedo reads 70 , my sat nav says I'm doing 63!!


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## tramp (May 25, 2005)

warning******

by Law speedo`s are callibrated so at athe given speed of say 30/50/70 you wont be going any faster than it shows on the speedo.

Very handy for the local traffic police and speed cameras, BUT a sat nav is not callibrated it uses those spinning things in the sky and takes the time to relay the distance to your position thingy so they can be out by as much as 4mph   especially when under heavy foliage or in built up areas.


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## selstrom (May 23, 2005)

I believe speedometers are required to have an accuracy of+ or - 10% and this is allowed for by the police when issuing tickets. Also they have to allow for the accuracy of their meter.

Before speedometers where digital the manufactures appeared to use some of the tolerance and set them to read faster. Now we have digital speedometers they seam to be more accurate. Both our vehicles are within 1% when checked with a stopwatch using the motorway distance markers over 10Km.


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## BlakeneyPlayer (Feb 22, 2009)

I think that it is still the case that the condition of the tyre will also affect the speedo readout-ie a new tyre will need slightly less revolutions to cover the same distance as a tyre that is worn down to 2mm-whereas the GPS reading will be unaffected (other than by the comments already posted.)
My experience is that the speedo reads a faster speed than the TomTom
Rob


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## TDG (May 26, 2009)

Don Madge said:


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


Speedo will always read a bit fast.
Sat Nav is very accurate *but* does not give an instantaneous reading but averages over a number of fixes. Depending on the system this can be a 4 second average or longer if you have bad signals or geometry


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## hblewett (Jan 28, 2008)

I understood that a speedo must be within 10%, but must never be slower than the true speed - i.e. correct to +10%. 

Our motorhome reads about 10% more throughout, but on the car it is within 1mph at 30 and about 5 mph at 60mph - the speedo always slower than the sat-nav. The sat-nav seems pretty steady at a steady speed (especially on cruise), but there is a time-lag in catching up with the true speed when accelerating or decelerating - due to this 'averaging' mentioned before. 

I tend to drive to the sat-nav (i.e. slightly quicker) - hope I'm right!


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## peribro (Sep 6, 2009)

As others have said, sat navs are very accurate except where the signal is obscured and when rapidly decelerating or accelerating. I have both a TomTom and a Road Angel and they invariably give the same speed as each other which in turn is about 6% less than the speedometer is giving me on our current vehicle. I have never driven a car where the speedometer is not overstating the actual speed by at least 5%.


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## Spacerunner (Mar 18, 2006)

I always use the garmin's speed reading when travelling in Europe.

Reason is Fiat in their infinite wisdom have decided to print KPH on the speedo in a dull red which is totally unreadable except when using a torch 3 inches from the dash.


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## fdhadi (May 9, 2005)

Like Spacerunner I use the Garmin's speed reading in Europe. Just switch it to show kpm's.


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## Don_Madge (May 1, 2005)

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" 8O 8O 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. 8O 

Safe travelling.

Don


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## Bernies (Mar 22, 2007)

This subject came up on the Simon Mayo Drivetime prog on radio 2 just the other week and they had an'expert' on who said speedos are required to be approximately within 10% correct, however, your satnav will always give the more accurate speed.

It's amazing what useless info I pick up on the way home....

Carol
Bernie's OH


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## short-stick (Jul 23, 2007)

Just a bit of useless information...
Volvo's "used" to build in a considerable bit of error so the speedo displayed a higher speed that that which you were travelling at. I had an early S60 and at an indicated 70mph I was actually doing 62mph! This was easy to prove, as there was a way of pressing certain button combinations to get the radio to display various inputs that the car was computing from, one of these was road speed in KPH. The displayed kph figure was accurate, the cars electronic mph figure overread. I did a trawl on the internet and found that the USA or Canada tolerance for speedo accuracy was much tighter than the UK, and in fact their Volvo speedo's were more accurate, it seemed it was possible for a correctly informed engineer to alter the accuracy level of the speedo on the car, howevr in the UK Volvo denied this! My later Volvo had a much more accurate speedo, as did my Ducato X250...


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