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Sat Nav speed vs speedo

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raybarrow
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Sat Nav speed vs speedo

#431524

Postby raybarrow » July 30th, 2021, 4:56 pm

Hi,

Just picked up my Fiesta (MOT and Service). While waiting, listened to a phone conversation with the service desk, like you do. Customer was unhappy that he had been 'done' for speeding - 10 mph over the limit. His complaint was that he was using the built in Sat Nav to monitor his speed. Why when he has a purpose built speedometer in front of him? I'm sure Sat Navs can only be near enough and then on a perfectly straight road with no hills up or down. I know I'm old school but Sat Navs navigate and Speedometers indicate your speed (I know they read under for safety reasons). Seems fairly straightforward to me.

Ray.

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Re: Sat Nav speed vs speedo

#431525

Postby pje16 » July 30th, 2021, 5:02 pm

Agreed your sat nav is likely to be more accurate than the cars speedo,
https://www.driving.co.uk/car-clinic/ca ... y-sat-nav/
except for a Fiesta perhaps :lol:

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Re: Sat Nav speed vs speedo

#431526

Postby bungeejumper » July 30th, 2021, 5:05 pm

That's a new one on me. Satnavs are always within a few mph of your true speed (due to uphill and downhill - they're very accurate on the flat.) Whereas speedos are allowed to be up to 10% optimistic, I think? But never to understate your speed.

Meanwhile, police speed traps have to allow a slight margin on the upside (for the motorist), or their results wouldn't stand up in court. It sounds to me as though Mr Grumpy was trying to rationalise the fact that he'd been rightly busted and was looking for somebody to blame.

BJ

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Re: Sat Nav speed vs speedo

#431538

Postby ReformedCharacter » July 30th, 2021, 5:43 pm

bungeejumper wrote: Whereas speedos are allowed to be up to 10% optimistic, I think? But never to understate your speed.

BJ

Yes, by design and known as 'flatter' :)

RC

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Re: Sat Nav speed vs speedo

#431544

Postby AsleepInYorkshire » July 30th, 2021, 5:55 pm

I believe car manufacturers set the speedometer to read about 3mph less than you are actually travelling. Has something to do with legal requirements not to have a speedometer that allows the driver to "unknowingly" speed.

The speed on the sat nav will reflect this. Unless of course Fiesta's have a "sat nav" recall issue :lol:

AiY

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Re: Sat Nav speed vs speedo

#431546

Postby swill453 » July 30th, 2021, 6:03 pm

AsleepInYorkshire wrote:I believe car manufacturers set the speedometer to read about 3mph less than you are actually travelling. Has something to do with legal requirements not to have a speedometer that allows the driver to "unknowingly" speed.

The speed on the sat nav will reflect this. Unless of course Fiesta's have a "sat nav" recall issue :lol:

As already said above, speedos invariably read more than you are actually travelling. You got the reason right though :-)

Scott.

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Re: Sat Nav speed vs speedo

#431573

Postby 88V8 » July 30th, 2021, 8:01 pm

The only time I doubt the speed on the satnav is when travelling beneath trees or in a tunnel.
In one of my vehicles I also have a GPS speedo, and that matches perfectly with the satnav.

V8

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Re: Sat Nav speed vs speedo

#431596

Postby jackdaww » July 30th, 2021, 10:45 pm

i have noticed that when travelling towards a speed indicator ( dont know what they are called ) - they light up as you approach - radar things - the speed shown is invariably identical to my satnav speed .

this happens every where i go . they cant all be wrong , my conclusion is satnav is very very accurate .

:)

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Re: Sat Nav speed vs speedo

#431599

Postby jfgw » July 30th, 2021, 11:10 pm

A couple of thoughts:

Was the conversation overheard incorrectly? Was the driver actually relying upon the sat nav to display the speed limit rather than the speed?

Was the signal lost? The sat nav may have been guessing.

I would expect most drivers to notice if their speed measuring device was 10mph out.


Julian F. G. W.

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Re: Sat Nav speed vs speedo

#431604

Postby Breelander » July 31st, 2021, 12:20 am

jackdaww wrote:i have noticed that when travelling towards a speed indicator ( dont know what they are called ) - they light up as you approach - radar things - the speed shown is invariably identical to my satnav speed .

this happens every where i go . they cant all be wrong , my conclusion is satnav is very very accurate .


I have two cars, one a classic Mini from the 60s, the other a modern VW Polo. Both of them when travelling towards one of those speed indicators have speedo reading exactly 10% higher that that shown by the radar trap. Cars may have changed a lot over the past 50 years - but that over-optimistic speedo is the one thing that hasn't :D

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Re: Sat Nav speed vs speedo

#431613

Postby jackdaww » July 31st, 2021, 7:13 am

Breelander wrote:
jackdaww wrote:i have noticed that when travelling towards a speed indicator ( dont know what they are called ) - they light up as you approach - radar things - the speed shown is invariably identical to my satnav speed .

this happens every where i go . they cant all be wrong , my conclusion is satnav is very very accurate .


I have two cars, one a classic Mini from the 60s, the other a modern VW Polo. Both of them when travelling towards one of those speed indicators have speedo reading exactly 10% higher that that shown by the radar trap. Cars may have changed a lot over the past 50 years - but that over-optimistic speedo is the one thing that hasn't :D


===========================

i have always thought it to be common knowledge that car speedos are 5-10% fast .

re satnav again , i dont know the thinking behind the reading varying when up or down hill . DAK?

also what is a GPS speedo ?

:?:

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Re: Sat Nav speed vs speedo

#431624

Postby bungeejumper » July 31st, 2021, 9:16 am

jackdaww wrote:re satnav again , i dont know the thinking behind the reading varying when up or down hill . DAK?

Basic triangular geometry, surely? If you're driving up a slope (i.e. the rising side of a triangle), your wheels are experiencing the "real-world" tarmac distances that you're travelling - even if your speedo isn't being too precise. ;) But your satnav, on the other hand, is giving you a view from space (so to speak), so it's measuring the true latitude/longitude of your progress - or, if you like, the horizontal base line of your triangle. Which will invariably be shorter than the stretch of tarmac that you're driving on. When you're back on the flat, the real-world tarmac experience will be closer to the view from space, and the calculation will come back into line.

Another reason why conventional speedos aren't accurate, of course, is that your tyres wear down, so that you get fewer and fewer centimetres per axle revolution as the tyre circumference gradually reduces. It isn't a big deterioration (maybe 3% over a 3mm thinning of your tyres?), but it all adds up.

BJ

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Re: Sat Nav speed vs speedo

#431625

Postby DrFfybes » July 31st, 2021, 9:18 am

88V8 wrote:In one of my vehicles I also have a GPS speedo, and that matches perfectly with the satnav.

V8


So you have 2 GPS speedo's that agree with each other? Reassuring at least :)

jfgw wrote:Was the conversation overheard incorrectly? Was the driver actually relying upon the sat nav to display the speed limit rather than the speed?

Julian F. G. W.


That does make sense - satnavs/Google maps rely on a database to display the speed limit, which can be out of date. The Avensis has a camera that reads (most of) the speed limit signs. It occasionally misses one, but it is scary how easily you can become reliant on these things and stop looking at the road signs, something I notice when I swap to the other cars.

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Re: Sat Nav speed vs speedo

#431627

Postby Watis » July 31st, 2021, 9:24 am

jackdaww wrote:
Breelander wrote:
jackdaww wrote:i have noticed that when travelling towards a speed indicator ( dont know what they are called ) - they light up as you approach - radar things - the speed shown is invariably identical to my satnav speed .

this happens every where i go . they cant all be wrong , my conclusion is satnav is very very accurate .


I have two cars, one a classic Mini from the 60s, the other a modern VW Polo. Both of them when travelling towards one of those speed indicators have speedo reading exactly 10% higher that that shown by the radar trap. Cars may have changed a lot over the past 50 years - but that over-optimistic speedo is the one thing that hasn't :D


===========================

i have always thought it to be common knowledge that car speedos are 5-10% fast .

re satnav again , i dont know the thinking behind the reading varying when up or down hill . DAK?

also what is a GPS speedo ?

:?:


Gradients will affect the satnav's speed calculation because you travel further to go a given distance. Driving from A to B over a hill requires you to drive a greater distance than along the flat. You'll need to travel further when going over the hill yet the distance between A and B hasn't changed (assume both journeys are straight lines) so your speed will be higher when the journey times are the same. In extremis, the satnav on a rocket taking off vertically will show zero mph, yet the rocket will be moving fast.

Watis

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Re: Sat Nav speed vs speedo

#431633

Postby pje16 » July 31st, 2021, 9:53 am

Watis wrote:Gradients will affect the satnav's speed calculation because you travel further to go a given distance. Driving from A to B over a hill requires you to drive a greater distance than along the flat.

Not necessarily
from the link I posted earlier
"However, there is usually some lag in the reading, and accuracy can be affected by GPS signal quality, bends in the road, inclines and speed changes. Fortunately, modern factory-fitted systems also use data from the car itself, integrating it with the GPS information so as to enhance the accuracy."

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Re: Sat Nav speed vs speedo

#431634

Postby swill453 » July 31st, 2021, 9:55 am

bungeejumper wrote:
jackdaww wrote:re satnav again , i dont know the thinking behind the reading varying when up or down hill . DAK?

Basic triangular geometry, surely? If you're driving up a slope (i.e. the rising side of a triangle), your wheels are experiencing the "real-world" tarmac distances that you're travelling - even if your speedo isn't being too precise. ;) But your satnav, on the other hand, is giving you a view from space (so to speak), so it's measuring the true latitude/longitude of your progress - or, if you like, the horizontal base line of your triangle. Which will invariably be shorter than the stretch of tarmac that you're driving on. When you're back on the flat, the real-world tarmac experience will be closer to the view from space, and the calculation will come back into line.

GPS triangulation works in 3D, so the height change will come into the equation, and it's capable of measuring the true speed on a slope.

(If the positioning was 100% accurate.)

Scott.

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Re: Sat Nav speed vs speedo

#431639

Postby bungeejumper » July 31st, 2021, 10:13 am

swill453 wrote:GPS triangulation works in 3D, so the height change will come into the equation, and it's capable of measuring the true speed on a slope.

(If the positioning was 100% accurate.)

Thanks, Scott, I've learned something I didn't know.

BJ

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Re: Sat Nav speed vs speedo

#431644

Postby raybarrow » July 31st, 2021, 10:22 am

Hi Folks,

Made a reply but seems to have dissapeared into the ether so I'll summarise. Problem of hearing only one side of a telephone conversation. On reflection I think the suggestion that the speed limit had altered and driver relied on Sat Nav indication rather than actual road signs.

Looks like I was wrong about the mph accuarcy of Sat Navs.

Although we have a Garmin Sat Nav (2012) I tend to use a combination of What Three Words and Here We Go or Waze these days. In reality having a good sense of direction it's generally the last few miles or in a strange town/city that I find 'Sat Navs' really usefull.

That customer will not get his inbuilt Sat Nav software update till later in the year, he'll just have to look at the road signs for a few months.

Ray.

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Re: Sat Nav speed vs speedo

#431646

Postby 88V8 » July 31st, 2021, 10:24 am

jackdaww wrote:...also what is a GPS speedo ?

As Dr FFybes hints, it's just another GPS device, like the satnav.
In this case it looks like a conventional speedo, with an analogue dial, but is not driven by the car. The GPS signal is use to drive a stepper motor that moves the needle.
Example here https://www.speedhut.com/gauge/GR338-GP ... ter-120mph

If you want to be really quirky and keep your original speedo when it would no longer be accurate due to a gearbox or diff swap, you can even get a GPS powered cable driver https://www.speedhut.com/ecommerce/prod ... e-Included)
Or of course you could just do some arithmetic and have your original speedo recalibrated, but let's not pour cold water on this more moderne solution.

raybarrow wrote:....I think the suggestion that the speed limit had altered and driver relied on Sat Nav indication rather than actual road signs.

Yes, I suspect that's bang on. Even though the satnav might bleep to warn of exceeding what it thinks is the speed limit, it might have stored an out of date limit. One still has to watch the signs.

V8
Last edited by 88V8 on July 31st, 2021, 10:28 am, edited 1 time in total.

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Re: Sat Nav speed vs speedo

#431647

Postby swill453 » July 31st, 2021, 10:27 am

raybarrow wrote:That customer will not get his inbuilt Sat Nav software update till later in the year, he'll just have to look at the road signs for a few months.

Yes, I've noticed that Google Maps satnav sometimes shows the speed limit. I guess the information it has is effectively crowdsourced, and if there is some doubt then it takes the cautious approach and doesn't give you information that might be wrong.

(I'd never fully trust it though, and certainly wouldn't blame it if I got a speeding ticket.)

Scott.


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