Speed Differences (1 Viewer)

[AdamB]

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!

 

[Landy Andy]

The van will be the most incorrect and I doubt the other two are that different to be honest. Assuming they are both using gps.

 

[Stealaway]

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

I find that the Sat nav is the most accurate. The van speedo is always a little slower

 

[Newfie]

How accurate is your car speedometer? #AskTCE

Have you noticed that your car speedometer reads faster than your satnav says? Find out why this happens in this week's #AskTCE at TheCarExpert.co.uk

www.thecarexpert.co.uk

 

[gus-lopez]

Sat nav I set my crusie to it in roadworks & cruise past the rest.

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[raysalaugh]

I find that the Sat nav is the most accurate. The van speedo is always a little slower

On mine the Sat nav says 50mph but the speedo reads 52mph.

 

[donkey]

I find that the Sat nav is the most accurate. The van speedo is always a little slower

How do you know

Geoff

 

[TheAleMan]

In the van on cruise I set the sat nav to 50 and the speedo reads 53 for instance.

 

[Spriddler]

I have three different methods to observe my speed.

That is your problem.
As Newfie has posted..........

 

[Stealaway]

How do you know

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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[donkey]

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?

Geoff

 

[Tigger-Tim]

The speedo is linked to rotations of the tyres. As others have said, it is only allowed to over-read by law.

At constant speed, in a straight line, a GPS based system will be almost 100% accurate.

 

[gus-lopez]

How do you know

Geoff

Because it is required by law to over read speed rather than under read.

 

[Oldrat]

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]

All my prev vans have always read approx 5mph more than the navs

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[pelican]

I find that the Sat nav is the most accurate. The van speedo is always a little slower

How do you know it is the most accurate?

 

[pelican]

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

 

[Stealaway]

How do you know it is the most accurate?

I refer the member to my previous answer

 

[Stealaway]

Don't you mean Gps reads slower

It could be

I know that the sat nav is more accurate than a speedo

 

[PaulOH]

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]

I always smile at comments such as "I bought this multimeter to compare against the van display, It is very accurate"? Says who?

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

Ian

 

[Lenny HB]

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.

 

[PaulOH]

I'm an old-schooler so!!!

 

[royandsharont]

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.

 

[Folkranger]

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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[rod_vw]

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.

 

[peterc10]

Sat Nav is the most accurate. Your speedo is designed to read up to 10% higher than you are actually travelling, but not lower at all.

 

[Andacami]

Stock speed readings on all vehicles will always read higher than actual physical speed as most have noted; proven with the GPS vs Speedometer deltas. This is normal, standard and legal.

 

[rod_vw]

Stock speed readings on all vehicles will always read higher than actual physical speed as most have noted; proven with the GPS vs Speedometer deltas. This is normal, standard and legal.

Yes.

 

[cumbriankeith]

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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