Speed Differences

[Oldrat]

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 but only up to 10% over

 

[AdamB]

Many thanks Funsters for this debate, I think it was enlightening and confirmed my belief, as with the majority of others, that the SatNav is the more accurate measurement for noting your 'true' speed.

Stay Safe
Drive Safe
Be Safe

 

[cyberyacht]

I have an "Amazon" GPS speedo. The vehicle one used to read about 3-4% faster. Took the motorhome for a run today and noticed that the dash and GPS were reading both the same. Perhaps it's the cold weather.

 

[NigelNovice]

I now have an app on my phone that uses GPS so I can tell what speed I am doing on my Lambretta as the standard ones are very erm vague I could also use the Google maps app and in Motorhome I use Sygic sat nav app to set cruise control to

 

[pappajohn]

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

Satnav will be more accurate than a Speedo BUT a Speedo can't legally read slower than true speed.
It can read fast by 10%

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[pappajohn]

I now have an app on my phone that uses GPS so I can tell what speed I am doing on my Lambretta as the standard ones are very erm vague I could also use the Google maps app and in Motorhome I use Sygic sat nav app to set cruise control to

GPS only goes up to 45mph then.

 

[Old Gibber]

The speedo can never be that accurate, because it depends on the number of rotations of the tyres within a particular time, and as the tyres wear the distance covered by each rotation will be that little bit less. Given the size of motorhome wheels the difference is quite significant.

 

[G-RMPS]

Also consider the mileage posts on the motorway are probably planted by reference to a satnav device. Maybe the days of the chap with a wheel and a counter on a stick have passed.

I believe that with clear visibility of the sky a satnav will be accurate to about 0.1mph at 60mph…but I’m not a lawyer

PS At higher speeds e=mc squared applies. Goole Einstein for details.

 

[Yorick]

Apps have no real credibility. Only as good as the programmer. I could write a speed app in a few hours. No idea how accurate it would be.

 

[G-RMPS]

True. Would make an interesting legal case. Meanwhile I guess I will trust that a GPS app developer has no motive for exaggerating (or unexaggerating!) speeds. I agree they could still make mistakes and the validity of their product output is only as good as their code testing regime.

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[Yorick]

True. Would make an interesting legal case. Meanwhile I guess I will trust that a GPS app developer has no motive for exaggerating (or unexaggerating!) speeds. I agree they could still make mistakes and the validity of their product output is only as good as their code testing regime.

AFAIK the proper GPS devices locks onto 9 satellites to get decent triangulation to find the location. A lazy programmer might only use 3 or 4, getting very loose locations.

 

[NigelNovice]

We had a traffic cop come to one our CPD days and he was on about speed, we asked about Sat Nav data and he could not come up with and official answer, it was also asked "why on an empty straight country road do we need to worry about speed limits, he said in case a Dear or something runs out in front of you

 

[JinxJetJock]

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!

Vehicle speedos are by design reading fast. I've always found the various sat-navs i have used have been more believable.

 

[kevenh]

Speedos are 2nd best compared to SatNav or other GPSr equipped devices for speed readings.
Why? Here's a handy highish level explanation on GPS: https://www.geotab.com/uk/blog/what-is-gps

 

[nicholsong]

We had a traffic cop come to one our CPD days and he was on about speed, we asked about Sat Nav data and he could not come up with and official answer, it was also asked "why on an empty straight country road do we need to worry about speed limits, he said in case a Dear or something runs out in front of you

I cannot imagine an Old Dear, grandmother type, running.

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[Tony the Manc']

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!

Sat nav always most accurate..........or so I am informed.

 

[Mleabon]

The GPS is the most accurate. One problem with vehicle speedos is that they have to work with different size wheels and tyres on the same car. So are probably a best fit. Mind you you would think that these days with computers controlling everything that it would be easy to adjust the speedo to suit the wheel / tyre combination.







PS I used to work for a company making GPS equipment



PS

 

[Furry Paws]

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

View attachment 708984

I think there is an unwritten rule of Plus 10% and 2 Miles Per Hour! So in a 30 Mph limit 10% equals 3 Mph plus 2 equals to 35 Mph, 36 Mph and you're nicked LOL

 

[robnchris]

Have trust in your Sat Nav, long before we had such a device in the car we used them whilst sailing, it was always a habit of mine on a changing tide to note how fast the speed log read compared to the speed from the Sat Nav.
For the non sailors, your yacht might be making 6 kts, compared to the Sat Nav reading of 0 kts.
The Yachts log is reading speed through the water, the Sat Nav is reading Speed Over Ground, (SOG) so you basically you are standing still.

 

[Trevorn]

One of the things regarding speedo readings is the number of people that claim there Speedo reads faster than they are travelling and drive at a higher indicated speed to compensate, then complain when they get fined for speeding. If you drive to your Speedo and you inadvertently drift over the posted limit (we all do it) you will more than likely be within the tolerance of any speed cameras.

Trevor

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[Dorwyn]

How do you know it is the most accurate?

In my case it's because I don't get flashed by speed cameras if I follow the sat nav's speed reading.

 

[kevenh]

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.

The speedo measures how fast you’re going and can be inaccurate (over reading). The distance you’ve driven is odometer’s job and that is accurate
Source

Edit: oh! It was ez to read that source once. The 2nd time a paywall popped up. Soz

 

[peterc10]

The speedo measures how fast you’re going and can be inaccurate (over reading). The distance you’ve driven is odometer’s job and that is accurate
Source

Edit: oh! It was ez to read that source once. The 2nd time a paywall popped up. Soz

I managed to read the source, but don't believe it. All the speedo and odometer do is count revolutions of the wheel. Both then multiply it by the nominal diameter of the wheel to calculate the distance travelled. The difference is that the speedo compares that distance of one revolution with time to calculate the speed, whereas the odometer just adds up the distances. Both will only be as accurate as the nominal diameter is to the actual diameter.

Having said that it wouldn't surprise me if modern cars use GPS to do that instead

 

[Bramidan]

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.

Mine is as well, Speedo and Garmin exactly the same.

 

[Widge]

Its a bit like a bakers dozen, you always were given 13 rolls, that way the baker knew that he had always given you slightly too much and could never fall foul of the law.

A speedo on a vehicle is made to account for inaccuracys due to mechanics and tyre wear pressures etc, but will never read overspeed, or the manufacturer could be held responsible, if you unwittingly exceeded the speed limit.

You have to use a mechanical system,, even though most new vehicles have some sort of gps, becouse GPS doesn't work very well in lots of situations like built up or mountainous areas tunnels etc.

Regarding the odometer, that might not be so inaccurate as it might only be the speed display either digital readout or needle that is raised slightly.

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[kevenh]

I managed to read the source, but don't believe it. All the speedo and odometer do is count revolutions of the wheel. Both then multiply it by the nominal diameter of the wheel to calculate the distance travelled. The difference is that the speedo compares that distance of one revolution with time to calculate the speed, whereas the odometer just adds up the distances. Both will only be as accurate as the nominal diameter is to the actual diameter.

Having said that it wouldn't surprise me if modern cars use GPS to do that instead

I think you’re right
I just assumed/didn’t expect the speedometer’s allowed inaccuracies to also apply to an odometer but unless newer vehicles have a new system, the same wheel revolution cable is the source of “info” for both counters.

 

[bigtwin]

In my case it's because I don't get flashed by speed cameras if I follow the sat nav's speed reading.

You are rather assuming that the speed camera is set to trigger bang on the speed limit. I very much doubt that that would be the case (particularly if we are to believe the oft peddled story that you are allowed x% + 2mph story).

Ian

 

[nicholsong]

The speedo measures how fast you’re going and can be inaccurate (over reading). The distance you’ve driven is odometer’s job and that is accurate
Source

Edit: oh! It was ez to read that source once. The 2nd time a paywall popped up. Soz

The last sentence in the 'Source' that you quote states that both speedometer and odometer can be inaccurate, which is logical in that both are reading information from the circumference of the tyres.

 

[Jeffr3y]

I use a speedo app on an old android phone because trying to read the tiny red Kph on the ducato dial is almost impossible and I can set the digital app display to mph or kph, whenever I've passed a illuminating speed sign they've agreed spot on, while the vehicle speedo has been showing high between 3mph at 30 to as far out as 7mph at motorway speeds.

 

[kevenh]

If the speedo has to be in the range of -0%/+10% the vehicle makers have to offset the reading to complie with this law.
The odometer is not offset so can be +/-5%, right?

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