Speed Gauge - Accuracy

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hornedav
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Speed Gauge - Accuracy

Post by hornedav » Wed Apr 17, 2019 9:59 am

I have a new 1.0 litre 85KW Polo Beats, with DSG. I'm wondering how accurate the Speed gauge is.

Does anyone know if modern VW under-estimate current speed? For example if the Gauge states 79 KPH...that it's really 75 KPH ?

Thanks in advance.

Griff74
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Re: Speed Gauge - Accuracy

Post by Griff74 » Wed Apr 17, 2019 10:16 am

All cars speedos over read as it is an offence to have one that under reads, tends to be by a small percentage so the amount it is out by increases with speed.

Andy Beats
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Re: Speed Gauge - Accuracy

Post by Andy Beats » Wed Apr 17, 2019 10:26 am

Download any of the free GPS speedometer apps that are available, then compare.
Mine over-reads by about 3-4mph at 70mph.
So I know I can sit at 75mph on my speedo and still be safe enough.
Checking speedo accuracy against a GPS app is one of the first things I do in any car so that I know if I'm actually breaking the law compared with the speedo.

vc-10
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Re: Speed Gauge - Accuracy

Post by vc-10 » Wed Apr 17, 2019 11:41 am

Waze includes a speedo, which will tell you if you're over the limit and by how much, and that works over Android Auto. Think it works over Apple CarPlay too.

Andy Beats
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Re: Speed Gauge - Accuracy

Post by Andy Beats » Wed Apr 17, 2019 11:46 am

Worst I came across was a Golf 1.6 petrol which we hired and was bang on the 10% maximum limit for accuracy.
You could tell something was wrong, an indicated 80mph just didn't feel like 80mph.
The car was actually only doing 72mph, an unacceptable margin of error to me.

hornedav
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Re: Speed Gauge - Accuracy

Post by hornedav » Wed Apr 17, 2019 12:49 pm

Thank you, for your tips and advice. Cheers. :)

monkeyhanger
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Re: Speed Gauge - Accuracy

Post by monkeyhanger » Wed Apr 17, 2019 2:37 pm

There has to be some level of over reading to account for tyre wear when you're counting no of wheel revolutions per second for speed.

Your typical R18 wheel on a Polo or Holf has a rolling diameter of about 630mm + tread. So 646mm on a new 8mm tread tyre and 634mm when down to 2mm tread.

That's a 2% diameter (and circumference) difference between a new tyre and a barely legal tyre. As the tyre wears, your speedo will be less accurate as the wheels spin more times to maintain the same actual (not indicated) speed.

Most speedos are around 4% optimistic on a new set of tyres.

stevereeves
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Re: Speed Gauge - Accuracy

Post by stevereeves » Thu Apr 18, 2019 2:43 am

A few times I've passed through speed limit display signs, say 30mph at an indicated 26mph, and the sign has flashed red + 'Slow Down', like I was speeding....

SRGTD
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Re: Speed Gauge - Accuracy

Post by SRGTD » Thu Apr 18, 2019 7:57 am

stevereeves wrote:
Thu Apr 18, 2019 2:43 am
A few times I've passed through speed limit display signs, say 30mph at an indicated 26mph, and the sign has flashed red + 'Slow Down', like I was speeding....
The issue could have been with the calibration of the sign, which I think are calibrated by the manufacturer or the sign, rather than the law enforcement agencies.

I’ve had the flashing sign happen a few times - I’ve been travelling at 28 in a 30 speed restriction area and the sign has flashed, indicating a speed in excess of 30mph.

Griff74
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Re: Speed Gauge - Accuracy

Post by Griff74 » Thu Apr 18, 2019 10:49 am

SRGTD wrote:
Thu Apr 18, 2019 7:57 am
stevereeves wrote:
Thu Apr 18, 2019 2:43 am
A few times I've passed through speed limit display signs, say 30mph at an indicated 26mph, and the sign has flashed red + 'Slow Down', like I was speeding....
The issue could have been with the calibration of the sign, which I think are calibrated by the manufacturer or the sign, rather than the law enforcement agencies.

I’ve had the flashing sign happen a few times - I’ve been travelling at 28 in a 30 speed restriction area and the sign has flashed, indicating a speed in excess of 30mph.
This is most likely, also the council tend to stick them in stupid places like on bends which messes up the accuracy of them. There is one by me that really under reads your speed as you approach it (on a bend) and then shows correct when you are almost upon it, thus making it pointless!

Andy Beats
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Re: Speed Gauge - Accuracy

Post by Andy Beats » Thu Apr 18, 2019 11:32 am

Griff74 wrote:
Thu Apr 18, 2019 10:49 am
This is most likely, also the council tend to stick them in stupid places like on bends which messes up the accuracy of them. There is one by me that really under reads your speed as you approach it (on a bend) and then shows correct when you are almost upon it, thus making it pointless!
At least your councils install them.
We wanted one for the village and was told we'd have to fund it ourselves.

stevereeves
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Re: Speed Gauge - Accuracy

Post by stevereeves » Thu Apr 18, 2019 2:23 pm

Speed cameras are everywhere in most towns (better cash cow locations), I just don't trust the blooming things. No, I don't have points on my licence and I'd like to keep it that way....

Andy Beats
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Re: Speed Gauge - Accuracy

Post by Andy Beats » Thu Apr 18, 2019 2:36 pm

stevereeves wrote:
Thu Apr 18, 2019 2:23 pm
Speed cameras are everywhere in most towns (better cash cow locations), I just don't trust the blooming things. No, I don't have points on my licence and I'd like to keep it that way....
The lines on the road are there as a second reference point for a reason, they know cameras can go wrong.
I don't agree they're cash cows, BTW.
Don't speed or, if you do, have the observational skills to notice the cameras.....simple as that. :!:

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