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Author Topic: Sat-Nav Speed Read Out.  (Read 941 times)

tickmike

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Sat-Nav Speed Read Out.
« on: October 24, 2017, 10:31:29 AM »

Putting smaller alloys on our New car makes the car speed reading 5mph slower than should be, any tips for getting a good reliable speed reading from the 'TomTom 620 Go' sat-nav ?. eg not driving under trees when taking a reading.
« Last Edit: October 24, 2017, 10:37:38 AM by tickmike »
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jelv

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Re: Sat-Nav Speed Read Out.
« Reply #1 on: October 24, 2017, 11:47:20 AM »

Haven't you got that the wrong way round? With smaller wheels the car speedo will be reading faster than your actual speed - and it will be a percentage not a fixed 5mph.

I found monitoring and remembering what the car speedo said on a dual carriage way or motorway when driving at a constant 50, 60, 70 as reported by the sat nav the best way.
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d2d4j

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Re: Sat-Nav Speed Read Out.
« Reply #2 on: October 24, 2017, 01:20:29 PM »

Hi

I hope you don’t mind but I think you both are wrong or right as there is not enough information to decide

If changing alloys to different size (16” to 15” or visa versa), but fitting correct tyre to each, so the overall wheel size remains same, then there should be no difference in speed

Also, sat nav are not calibrated so the speed cannot be relied upon as an accurate speed measurement

Many thanks

John
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d2d4j

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Re: Sat-Nav Speed Read Out.
« Reply #3 on: October 24, 2017, 01:21:23 PM »

Oops sorry, one more point, you must inform insurance company off any changes or you could void your insurance

Many thanks

John
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tickmike

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Re: Sat-Nav Speed Read Out.
« Reply #4 on: October 24, 2017, 03:24:52 PM »

Buying a new Vauxhall Mokka X, Wheel size 215x60x17" 96H.
 I found out after we had paid for the car, the road wheel will not fit in the 'Wheel Wheel' in the boot.  :no: 

For the spare, I bought a 16" steel wheel and fitted a 215x55x16" 96H tyre and that fits in the wheel well and is a H (130MPH) rated tyre.

Just think you have 4/5 people in the car with a boot full of luggage you get a flat, change it for the spare what are you supposed to do the the road wheel, ask your passengers to have it on there nee  ;D  :(

Vauxhall CEO is going to get an email soon from me. >:D

So I bought 4 new 16" alloys and they now will fit in the wheel well.

Using this on-line tool   http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

Says 2 MPH slow at 30MPH, I am getting 5 MPH slow against the sat-nav. !
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d2d4j

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Re: Sat-Nav Speed Read Out.
« Reply #5 on: October 24, 2017, 03:40:19 PM »

Hi

That’s definitely a smaller wheel, but you changed the rim and tyre to smaller

I would ensure you have informed insurance company of change

Our car did not have a spare wheel, just one of those puncture sealant kits.

It is worth noting though, if you are with AA or RAC (others may also offer same), and have a puncture. They carry correct spare wheel to get you going and you repair your tyre, then they collect loan wheel

Many thanks

John
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Ronski

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Re: Sat-Nav Speed Read Out.
« Reply #6 on: October 24, 2017, 04:21:22 PM »

Many cars do not now have spares, some have space savers, only if you're really lucky do you get a full size tyre and somewhere to store it. Some cars we have pass through have huge 22" wheels, we removed some 315/35R20 from a company X5 to reduce cost, no idea where you'd put one of those!

You've gone lower on the wheel diameter, and a lower profile, the profile is the height of the tyre as a percentage of the width. End result is the rolling circumference will be quite a bit less.

As John states you do need to inform the insurance company, as it counts as a modification and they could refuse to payout if any claim was made and it came to light.

I have a VW Tiguan and the speedo needle reads about 5 mph faster at around 60 than I'm doing. My dash has a digital option for displaying the speed and this tallies with the GPS in my phone. IIRC the speedometer can read faster than you're going (within certain limits) but not slower. You can get various phone apps, some that even use an external GPS which is a  lot more accurate.

Another thought is if the tyre size you have gone with is standard fitment option for that vehicle then the main dealer may be able to change the settings to suite that tyre size.

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4candles

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Re: Sat-Nav Speed Read Out.
« Reply #7 on: October 24, 2017, 06:16:57 PM »

Also, sat nav are not calibrated so the speed cannot be relied upon as an accurate speed measurement


The satnav derives its time from the GPS satellites, and calculates its position by triangulating the time of reception of the signal from three or more of them. If the device in question accurately displays its actual location, then I can't see why any 'calibration' is required to display an accurate speed.
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d2d4j

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Re: Sat-Nav Speed Read Out.
« Reply #8 on: October 24, 2017, 06:51:40 PM »

Hi

Many thanks

Please see this link or google sat nav accurate speed to show why sat nav speed cannot be relied upon

Many thanks

John

https://www.driving.co.uk/car-clinic/car-clinic-which-should-i-trust-my-speedometer-or-my-sat-nav/
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sevenlayermuddle

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Re: Sat-Nav Speed Read Out.
« Reply #9 on: October 24, 2017, 07:08:13 PM »

If the new wheels and tyres now fit the spare wheel well, and the old ones didn’t, then the new ones must be smaller.  Hence speedometer will over-read.   Be very mistrustful of anything on the web relating to ‘equivalent’ tyres & wheels, there’s an awful lot of dangerously inaccurate data published.

Also worth bearing in mind, no speedometer that is driven from wheel sensors can be very accurate, at least not all the time, simply because it will drift by a percent or two as the tyre tread wears down (less tread means smaller circumference). Legally they are allowed to over read a little, but must never under read.   

I think satnavs can usually be trusted, I suspect that Sunday times article is just plain wrong about how Sat Navs work, in relation to speed measurement.   I would assume they measure speed by detecting phase shifts and Doppler changes in the Satellite signals rather, than repeated location sampling.

As others have said, make sure your insurers know, else you might be accused of ‘non disclosure’ if you ever need to claim.   They may want to increase your premium, the argument often is “different wheels might make the vehicle more attractive to a thief”.
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jelv

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Re: Sat-Nav Speed Read Out.
« Reply #10 on: October 24, 2017, 09:22:36 PM »

Please see this link or google sat nav accurate speed to show why sat nav speed cannot be relied upon

https://www.driving.co.uk/car-clinic/car-clinic-which-should-i-trust-my-speedometer-or-my-sat-nav/

This totally endorses the suggestion I made earlier:

I found monitoring and remembering what the car speedo said on a dual carriage way or motorway when driving at a constant 50, 60, 70 as reported by the sat nav the best way.

On a straight road with no issues the Sat Nav will be more accurate as the article you linked suggested.

In my car I've found that driving at 63 mph as reported by the car speedo (and cruise control) I'm doing 60 by the Sat Nav so that's what I remember.

I suspect the speedo in cars is calibrated to the largest wheel/tyre size allowed (including nobblies and winter tyres which tend to be bigger). That way they will never under report the speed.
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4candles

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Re: Sat-Nav Speed Read Out.
« Reply #11 on: October 24, 2017, 10:25:33 PM »


Please see this link or google sat nav accurate speed to show why sat nav speed cannot be relied upon



Agreed, it cannot be relied on if travelling a bendy road, an incline, or the signal is shielded - that's surely a given.
I was just querying the necessity for calibration.
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d2d4j

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Re: Sat-Nav Speed Read Out.
« Reply #12 on: October 24, 2017, 10:51:57 PM »

Hi 4candles

Many thanks

To calibrate should give accurate reading and not one which should change depending on circumstances. In this instance of sat nav, it maybe accurate on straight level roads, but not accurate as described in others, therefore it cannot be calibrated fully, whereas the Speedo of a car can be calibrated to a high level of their accuracy chosen.

Beyond that it could becomes opinionated with sat nav

This is my view and apologies if you disagree.

Many thanks

John
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sevenlayermuddle

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Re: Sat-Nav Speed Read Out.
« Reply #13 on: October 24, 2017, 10:57:44 PM »


Agreed, it cannot be relied on if travelling a bendy road, an incline, or the signal is shielded - that's surely a given.
I was just querying the necessity for calibration.

Many modern devices are crammed full of accelerometers, gyroscopes and compasses that allow a pretty good guess at velocity, even in black spots.   Eg, the car was doing 20mph North before losing sight of satellites, then it turned West by 20 degrees, then it decelerated at 0.1G for 3 seconds, so now it is doing zzz mph. :)

As mentioned, pretty sure the basic speed comes from Doppler shift rather than repeated location sampling.   The difference is, speed measured from Doppler can still be accurate even when location is inaccurate, and also it is not affected by bendy roads. 

If you search you will find countless discussions on countless forums, debating whether Sat Navs use Doppler for speed measurement.    Opinions differ, but my money is that they do. :)

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4candles

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Re: Sat-Nav Speed Read Out.
« Reply #14 on: October 25, 2017, 12:46:43 PM »

@ d2d4j
No apology required.   :)
I think we're broadly in agreement - that in ideal conditions a satnav will accurately display one's speed, and that there is nothing we can do to maintain that accuracy when conditions are not ideal.


@ sevenlayermuddle
I don't have the hours available to study the subject in depth, but reading around various sites, I get the impression that Doppler was the predominant method used by earlier systems such as the US Navy's Transit (NAVSAT), whereas the current GPS system uses it in an auxiliary correction/confirmation way - for want of a better phrase.


This seems to be one of the most authoritative sources -  https://www.colorado.edu/geography/gcraft/notes/gps/gps.html
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