# Accuracy of speedo and computer



## Rapide561 (Oct 1, 2005)

Hi

I was not sure of the best place within the forum whether here, base chassis or tech etc. 

Well here goes....

On board computer showed 350 miles and 25.4 mpg for the trip. I refuelled at 65.00 litres and thus had 24.48. Allowing for experimental error, I think this is a fair cop 

Speedo - well - this seems to be "fast". If I set the speedo at 100km/h/62 mp/h, coaches overake me. The SatNav suggests I am going about 57 mph. This concurs with the fact that most coaches are limited to 62mph/100 kp/h. 

At 70 on the clock, the satnav indicates 64 mph. 

Would you think the speedo or the sat nav would be the accurate one?

Russell


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## pete4x4 (Dec 20, 2006)

Sat nav is very accurate as long as you have at least 4 sats in site. I can't remember the exact figure but it is 0.x% accurate.
Go wit the Sat nav everytime


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## Ian_n_Suzy (Feb 15, 2009)

Hi,

I get very similar "diffence of speed" between my Speedo and Sat Nav. I have no idea which one is likely to be the most accurate though.


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## lucy2 (Jun 27, 2007)

I always go by the sav for speed, I was told at 50+mph vehicles speedos can be 10% out


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## GerryD (Sep 20, 2007)

Speedo can show up to 10% faster than is true, but cannot show slower than true speed. 
So at indicated 60mph the correct speed would be between 54mph and 60 mph, but never more.
Sat Nav relies on triangulation, so will be absolutely accurate.
Gerry


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## dbh1961 (Apr 13, 2007)

Sat Nav is much more accurate than speedo, but not perfect. It takes no account of gradient, so on a hill, it will simply 'measure' distance as the crow flies, and could under measure distance, and hence understate speed. So if you go down a hill in a 30 limit, with the satnav saying 30, you could be doing more.


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## ob1 (Sep 25, 2007)

As GerryD says, speedo's are deliberately made to read faster than you are actually travelling to allow the manufacturers a tolerance margin on the true figure. 

On board computers are also less than accurate in practise in their consumption readings. Our current one, and the previous one, both understate the actual consumption figures by approx 3mpg.


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## RichardnGill (Aug 31, 2006)

I agree a Sat nav is very good for speed figures but what about if you are on a steep hill? 

Does it compensate for the extra distance travelled compared to flat land? I have often wondered that, I guess it will not make much difference and still be much closer than a speed. 


Russell. 

I find our on board computer to be between 0.2 and 1 MPG out. 


Richard...


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## dbh1961 (Apr 13, 2007)

No, it doesn't compensate for hils
Yes, it is still more accurate

BUT - unlike a speedo, it can UNDERREAD



RichardnGill said:


> I agree a Sat nav is very good for speed figures but what about if you are on a steep hill?
> 
> Does it compensate for the extra distance travelled compared to flat land? I have often wondered that, I guess it will not make much difference and still be much closer than a speed.
> 
> ...


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## timbop37 (Jun 28, 2006)

My suspicions have always thought about 10% but I have been told that car companies deliberately calibrate them about 4mph higher than actual speeds. This seems to be confirmed with my sat nav, and some of the digital displays that flash your speed at the side of the road.

I would go with your sat nav.


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## cabby (May 14, 2005)

For a correct speed reading the sat nav is more accurate, however if you rely on your speedo then you will not get any tickets for speeding. :roll: :roll: 

cabby


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## timbop37 (Jun 28, 2006)

cabby said:


> For a correct speed reading the sat nav is more accurate, however if you rely on your speedo then you will not get any tickets for speeding. :roll: :roll:
> 
> cabby


Sounds like a lesson gleaned from personal experience, Cabby.


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## asprn (Feb 10, 2006)

dbh1961 said:


> Sat Nav is much more accurate than speedo, but not perfect. It takes no account of gradient, so on a hill, it will simply 'measure' distance as the crow flies, and could under measure distance, and hence understate speed. So if you go down a hill in a 30 limit, with the satnav saying 30, you could be doing more.


I've learnt something today. 8O I'd never thought about this point, and I can quite see it.

Sheesh.

Dougie.


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## SaddleTramp (Feb 15, 2007)

I think the point of speed when going up or down hills will be so minute as not to matter at all, Look at the distance from the earth the sats are, Also the sats are not directly above you and if you are working on angles this is what has to also be taken into consideration.

After all if you are going up a 9% gradient the Sat could be directly facing the hill ie behind you and above (obviously) so in actual fact the speed "Could" be exact, and the same on the going downhill, and using the very same argument you could be at a worse angle on the level.

Sat Navs are far more accurate than Speedo's a speedo is ALWAYS at least 10% fast


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## 125678 (Jul 13, 2009)

*speed gps*

yes go with your speedo for safty cams ( bless ) as 10% fast is a true gauge, sat navs i have 2 and would say true speed.

the onboard comp i would say is true reading as i have done the old fill it to the top and refill at the same fuel station etc.

it is a shame the fuel is not as good as the mh ci 656 2.3 fiat 22 mpg mid range, 70mph + around 20/21 mpg, driving around 46 on gps 24 + with wind behind you,,,,, but you will not have any friends on the motorways.

i would be interested if any one was getting better as i have had my one check out at my service dealer.

reb


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## Boff (May 10, 2005)

Hi!



timbop37 said:


> My suspicions have always thought about 10% but I have been told that car companies deliberately calibrate them about 4mph higher than actual speeds.


EU regulations say that a speedo *may never underread*, but may overread up to *10% +4kph*. So better check your individual speedo's overread, before relying upon it.

Regarding sat-nav speed displays: The GPS receiver in a sat-nav actually delivers 3-dimensional coordinates, latitude, longitude, and height. So it would be possible for the sat-nav software to compensate for gradients when calculating speed. And it seems to me that at least my TomTom does just that.

Best Regards,
Gerhard


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## Annsman (Nov 8, 2007)

I think this is one of those, in the grand scheme of things, pointless discussions that facinate us boys! Why on earth does the "missing 10%" matter a midges thingy when we are driving along in our vans! If it was vital to plan a mission down to the last minute then fair enough, but come! on who on earth is that fussed really?

Some interesting points being made though!


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## tramp (May 25, 2005)

hi all,
this probably explains why when in my lorry with the very accurate cruise control/limiter set at 56mph I always catch caravans etc crawling at 55mph :lol: .

And they look at you like you are doing mack 1 :roll: 

Its a shame that vehicle manufacturers cant make them more accurate as like coaches and lorries with limiters /tacho graphs fitted are required to be accurate by european laws  

tramp


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## TDG (May 26, 2009)

Boff said:


> Hi!
> 
> 
> 
> ...


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## skiboycey (May 21, 2009)

Regarding the discussion on GPS speed and gradients the difference is insignifficant. 

For my pains I compete in gliding competitions at World Level (I spend 6 months every year travelling to do this) and we rely greatly on GPS technology to calculate final glide angles and speed vectors incorporating head/tailwinds etc. If you do the calcs (Right angled triangles, pythagorus' therom and a few sin/tan/cosin stuff from 'O' level maths) you'll discover that at glide angles (or hills) flatter than about 4 to 1 and at the speeds we are travelling gliding (around 70kph so slower than a motorhome) the inaccuracy is less than 1% and this gets less as the slope gets flatter and the speed increases. A quick calculation in my head for a motorhome going 100kph on a steep 1 in 8 slope (this is very steep for a big road) would see the inaccuracy due to glide angle as being significantly less than the inaccuracy inherent in the instrument itself unless you are using expensive and complex differential satellite technology. We, indeed, use this and it's still not worth bothering to include the slope in a speed calculation at speeds over 50 KPH and slopes less than 5 to 1.

So, in short, the GPS is reading the speed correctly for all reasonable circumstances and your van speedo will read over so that you cannot legally attack the manufacturer if you get a speeding ticket and the speedo said you were going slower than the limit (the REAL reason they read over)...

Incidentally GPS is a lot less accurate than claimed. We cannot use it to score competitions at less than ten second accuracy as each unit calculates an internal time depending on which satellite it finds firsts, nor in practice are they accurate to much less than about 10 metres which is fine if you want to guide a bomb in (what it was designed for) but not so good to try to separate a bunch of highly competitive pilots all arriving at the finish line very close together. The whole shebang is also very dependant on where the satellites are and if your unit can pick up the differential satellite which sits over the mid-atlantic. Only newer and more complex units can do this. Cheaper ones like road navigation devices 'fudge' the data and put you on the nearest road giving the impression they are always completely sure where you are without actually really knowing!

Cheers, Mark


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## Spacerunner (Mar 18, 2006)

FWIW

When touring France I found that the Km scale on the speedo was so indistinct that I used the KPH reading on the Garmin. 
Seemed to work just fine as I never got a speeding ticket!


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## TDG (May 26, 2009)

[quote="skiboycey" Cheaper ones like road navigation devices 'fudge' the data and put you on the nearest road giving the impression they are always completely sure where you are without actually really knowing!

Cheers, Mark[/quote]
Had a good Garmin example of this in France recently.
Beause we were late I decided to take to the auto route which ran sort of parallel to the planned N route. However she showed me as being on the N route for about 10 minutes until the road had diverged about 0.5 km and I failed to obay(again) one of her commands


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## Boff (May 10, 2005)

Hi!



skiboycey said:


> Regarding the discussion on GPS speed and gradients the difference is insignifficant.


After having done the math now I come to the same conclusion:

On a 1:8 slope the gradient error is below 1%, so can be neglected. On a 1:4 slope the error would be about 3%, so just the same as the tolerance of a speed cam. But, who would do speeding on a 1:4 slope. With a motorhome! 8O



tramp said:


> Its a shame that vehicle manufacturers cant make them more accurate ...


They can. Modern speedos could easily be adjusted to better than 3-4% tolerance, while still excluding underreading. However, they would attract an avalanche of complaint letters from irate drivers having been caught speeding because they kept following the traditional "+10%-is-safe rule".

Best Regards,
Gerhard


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## framptoncottrell (Jan 6, 2006)

I agree with Gerhard - on a 1 in 10 gradient a GPS showing 30mph would actually mean 30.15mph on the road.

What you have to be careful with GPS velocity is that it is an average over, I believe, 60m - that is why car manufacturers don't fit GPS speedos since the mechanical types give instantaneous speed which GPS cannot do.

Dr (mathematical, not medical) Roy


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## asprn (Feb 10, 2006)

skiboycey said:


> A quick calculation in my head for a motorhome going 100kph on a steep 1 in 8 slope....


lol - if my motorhome was doing 100kph down a steep 1 in 8 slope, I probably wouldn't be wondering about whether my GPS was compensating for the gradient or not. :lol:



SaddleTramp said:


> Sat Navs are far more accurate than Speedo's a speedo is ALWAYS at least 10% fast


First point, usually correct. Second point, not true. I have an old Bedford CF tipper whose speedo is *exactly* spot-on with my TomTom. My TomTom in turn is exactly spot-on with the calibrated speedos in the cars I use at work. God knows why the truck's speedo is so accurate - it only does 55mph flat out! 

Dougie.


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## Stanner (Aug 17, 2006)

asprn said:


> God knows why the truck's speedo is so accurate - it only does 55mph flat out!
> 
> Dougie.


Probably find the wheels/tyres are no longer the original size and the rolling radius has changed.


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## asprn (Feb 10, 2006)

Stanner said:


> Probably find the wheels/tyres are no longer the original size and the rolling radius has changed.


Thought of that, checked that, not the case. It's probably a manufacturing error, in that they accidentally made it accuraute. 

Dougie.


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## timbop37 (Jun 28, 2006)

I actually wonder if most trucks are more accurate, perhaps something to do with tachographs, etc, of which I know nothing about. :roll:


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## Rapide561 (Oct 1, 2005)

*Speed*

Hi

As far as I know, coaches and lorries go to somewhere like "Lucas Service" for their respective speed limiters calibrating.

Russell


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## Annsman (Nov 8, 2007)

No seriously guys, that's enough! Not three pages! :lol:  8O :wink:


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## poleman (Aug 31, 2008)

*accuracy of speedo*

Your correct Russel Tachograghs have to been calibrated every six years and rolling road checked every two. Checked with a machine at MOT as well any more than 1km over and it fails, they are very accurate. 
My speedo and satnav give very different reading and was not sure what to believe until I was flashed by a camera in Scotland sat nav reading 52mph speedo closer to 60mph, no ticket though thankfully.
Sat nav speed 57mph just catch trucks and pass :lol:


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## strod (Sep 2, 2007)

*speedo v satnav*

Surely the way to protect ones driving licence and to avoid this all controlling money grabbing government from robbing one of more cash,is to rely on the vehicle speedometer. Thought of a decal on the rear of motorhome "give us back our free speech - give us back our freedoms;I would probably get apprehended for breach of the peace or worse under the terrorism laws. Anyway best wishes to all our fellow travellers. EdLyn


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## Autoquest (May 16, 2007)

Annsman said:


> I think this is one of those, in the grand scheme of things, pointless discussions that facinate us boys! Why on earth does the "missing 10%" matter a midges thingy when we are driving along in our vans! If it was vital to plan a mission down to the last minute then fair enough, but come! on who on earth is that fussed really?
> 
> Some interesting points being made though!


I set my Cruise for 55mph on a 4 day 1000 mile trip to the med, sat nav gave me a true speed of 48mph - no wonder each day's driving was a little longer than expected... Now set the cruise speeds to sat nav speeds for a true 55mph.


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## RichardnGill (Aug 31, 2006)

48MPH, Just think of the fuel you saved though  


Richard...


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## DTPCHEMICALS (Jul 24, 2006)

Thanks Russell i often thought of posting this as all my vehicles are "slower " than tom tom


dave p


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## 106559 (Aug 19, 2007)

DTPCHEMICALS said:


> Thanks Russell i often thought of posting this as all my vehicles are "slower " than tom tom
> 
> dave p


 It's common knowledge. :roll:


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## Carl_n_Flo (May 10, 2005)

Annsman said:


> No seriously guys, that's enough! Not three pages! :lol:  8O :wink:


NOPE!!!!!!! FOUR...............................and counting........... :lol: :lol: :lol:


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## 106559 (Aug 19, 2007)

Might just squeeze 5 out of this yet. :lol:


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## Stanner (Aug 17, 2006)

5?


5 What?

Hope this helps.


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## Steamdrivenandy (Jun 18, 2007)

In these nearly '5' pages nobody's mentioned the frustration of bobbling along in a long 50mph set of dual carriageway roadworks and getting held up by two righteous, but non satnav bimbles doing 45mph cos their speedo's say 50!

Grrr

SDA


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## RichardnGill (Aug 31, 2006)

Yep Any 

I curse the 45MPH mob in road works, especcially the 14 miles one's on the M1. 

Up to 5 yet? 


Richard...


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## Rapide561 (Oct 1, 2005)

*Speed*



Steamdrivenandy said:


> In these nearly '5' pages nobody's mentioned the frustration of bobbling along in a long 50mph set of dual carriageway roadworks and getting held up by two righteous, but non satnav bimbles doing 45mph cos their speedo's say 50!
> 
> Grrr
> 
> SDA


Try driving on the M1 between J28 and J25 - 15 miles of it!

Russell


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## Steamdrivenandy (Jun 18, 2007)

*Re: Speed*



Rapide561 said:


> Steamdrivenandy said:
> 
> 
> > In these nearly '5' pages nobody's mentioned the frustration of bobbling along in a long 50mph set of dual carriageway roadworks and getting held up by two righteous, but non satnav bimbles doing 45mph cos their speedo's say 50!
> ...


Haven't been down that part of the M1 for nearly a year Russell.

What on earth are they doing to it that involves 15 miles of roadworks?

SDA


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## Rapide561 (Oct 1, 2005)

*Road works*

Well there is a lot going on - basically making the life of the motorist a misery, standing around leaning on a brush, driving a lorry with hazard lights on and erm, well, I don't know what is officially going on. Resurfacing and possibly widening to four lanes. Not sure.

Russell


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## sallytrafic (Jan 17, 2006)

Sorry but the gps system does work in 3D its just the mapping that doesn't at the moment but its coming soon to a gps near you.


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