Speed Differences (1 Viewer)

Mar 21, 2020
32
122
Frome, Somerset, UK
Funster No
69,535
MH
N&B Arto 69GL (LHD)
Exp
Since 2005
Morning all,

This must have been discussed before but I can't find the thread.

When I'm travelling, of course I keep the the speed limit of my Campervan (N&B Arto 69GL 2005) on public highways! However, I have three different methods to observe my speed: the speedometer, my SatNav (Garmin Camper) and an App on my iPhone (plus an additional speed if I pass a street speed trap).

Question: At a given speed all three state different values, but in your Funsters opinion, which one is correct?

Answers on a postcard, no conferring!
 
Dec 24, 2014
9,318
48,082
Hurstpierpoint. Mid Sussex.
Funster No
34,553
MH
Compass Navigator
Exp
Ever since lighting was by Calor gas.
I have three different methods to observe my speed.
That is your problem.
As Newfie has posted..........

1674406789533.png
 
Last edited:
Jan 8, 2013
8,496
11,538
Dronfield - Derbyshire
Funster No
24,202
MH
Burstner Lyseo 690G
Exp
Happy FLT since 2011
How do you know :unsure: :unsure:

Geoff
Every vehicle speedo I have had my Tom-tom satnav in - reads slower.
This new van came with a built in Pioneer sat nav and that one always reads faster than the vans speedo as well.
I trust the satellites more than a mass produced cheap van speedo ---- that might even have different tyres than standard

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Nov 13, 2011
1,486
3,544
Lincolnshire
Funster No
18,889
MH
PVC
Exp
30 years
Every vehicle speedo I have had my Tom-tom satnav in - reads slower.
This new van came with a built in Pioneer sat nav and that one always reads faster than the vans speedo as well.
I trust the satellites more than a mass produced cheap van speedo ---- that might even have different tyres than standard
I think you may be right.
I always smile at comments such as "I bought this multimeter to compare against the van display, It is very accurate"? Says who? :rolleyes:

Geoff
 
Oct 24, 2020
580
2,431
Funster No
77,187
MH
VW
Exp
Since 2017
Because it is required by law to over read speed rather than under read.
So by your own post it’s inaccurate.

My police motorcycle had 2 speedometers.
The OEM speedo and a calibrated AT head 5” speedometer.
I used to check the AT speedo each tour of duty and it was always accurate, or reported defective.

The BMW speedo however always read 36 mph at a true 30mph.

I’m a straight line, the Sat Nav will probably be the more accurate, that’s the one I prefer to depend upon
 

Tombola

LIFE MEMBER
Nov 21, 2020
4,915
16,005
Merseyside
Funster No
78,053
MH
Rapido 8094DF
Exp
Since 2004
All my prev vans have always read approx 5mph more than the navs

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May 29, 2014
2,082
1,708
southampton
Funster No
31,701
MH
A frame
Exp
since 1985
Every vehicle speedo I have had my Tom-tom satnav in - reads slower.
This new van came with a built in Pioneer sat nav and that one always reads faster than the vans speedo as well.
I trust the satellites more than a mass produced cheap van speedo ---- that might even have different tyres than standard
Don't you mean Gps reads slower
 
Feb 6, 2019
406
932
Santa Maria de Palautordera, Barcelona, Spain
Funster No
58,389
MH
LMC Liberty 560 1992
Exp
Since 2017
Motoring Which? used to assess the accuracy of speedometers on cars they tested - maybe they still do - but it was normal for them to say that the speedo was up to 10% optimistic - which is worrying because it means that for every 100miles, the mile counter is clocking up 110.

My Ducato broke it's speedo cable a couple of years ago and I couldn't persuade local garages to change it - 'a specialist job' they said - so I sourced a part online (same as a Lada apparently) and managed to find a (written) description ion a website telling how to change it, and then did so. It was a 3hour job with lots of swearing and rolling around underneath trying to see things reflected in a mirror, but it eventually worked.

Now, when the speed monitoring camera at the end of my street says I'm doing 32kph, I look at the speedo and that's exactly what it's displaying.

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bigtwin

LIFE MEMBER
Oct 29, 2009
5,290
10,034
Derby
Funster No
9,111
MH
Concorde
Exp
Since 2006
I always smile at comments such as "I bought this multimeter to compare against the van display, It is very accurate"? Says who? :rolleyes:

Indeed, many misinterpret resolution (eg 29.7mph) for accuracy (eg 29 mph).

Ian
 

Lenny HB

LIFE MEMBER
Oct 18, 2007
54,030
152,365
On the coast in West Sussex
Funster No
658
MH
Hymer B678 DL
Exp
Since 2008 & many years tugging
Satnav will be accurate providing you are on a dead straight level road any variation in straightness or height and accuracy will vary but it will still be far more accurate than your speedo.

The spec for speeds is - 0/+10 +10kph(6mph) for the life of the vehicle.

The speedo over read does not affect the accuracy of the odometer, it used to on old very old vehicles with cable speedos.
 
May 14, 2021
1,756
2,818
East Yorkshire, UK
Funster No
81,218
MH
ADRIA Twin 640SLB
Exp
Since 2012
Ironically I have found the vehicle speedo to be quite a bit higher reading than my sat nav’s over the years, until this new 22 Ducato. Not sure if the auto makes a difference but the Speedo is exactly the same as the sat nav.
 
May 2, 2014
2,255
4,617
Washington
Funster No
31,281
MH
Hymer B 584DL
Exp
17 years + 35 tugging
Whenever an industry is given a legal tolerance, the businesses within that industry will always take advantage of the tolerance to either save money by putting less in or to flatter the specification by appearing to give more. speedo's therfore always read higher.

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Jan 17, 2014
1,270
2,401
Wellington, Telford, Shropshire
Funster No
29,731
MH
Van Conversion
Exp
Since 1977
Use the motorway marker posts and a stop watch to prove/calibrate your speedo. With some vehicles (VW for instance) it is possible to alter the speedo calibration with coding via the ODBII port but only by minor amounts either way.

I have a dashcam that has a speed readout on screen which always matches roadside speed displays and calculates from GPS signals.
 
Jun 20, 2013
44
49
Kendal
Funster No
26,574
MH
Pilote P650GJ
Exp
since 2000 (a couple of VW campervans and a low profile Dethleffs)
Use the motorway marker posts and a stop watch to prove/calibrate your speedo. With some vehicles (VW for instance) it is possible to alter the speedo calibration with coding via the ODBII port but only by minor amounts either way.

I have a dashcam that has a speed readout on screen which always matches roadside speed displays and calculates from GPS signals.
I was meticulous/curious/sad enough to actually do this, timing and counting the motorway distance marker posts while cruise was deployed at several different speeds, on an otherwise tedious journey some time ago. I can report that unsurprisingly GPS is really rather accurate! From other experiences I know that GPS can be unreliable when in heavily wooded areas or amongst tall buildings - again unsurprising given that both of those factors prevent the GPS device from "seeing" enough satellites to get good fixes.

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