speedo accuracy

[laney]

I have a TomTom Go and a Road Angel, and if I use them together at say 80 mph they both read the same, but the car reads 73 MPH, I read on an American forum that VW issued a service bulletin for this problem, however my dealer knew nothing of it.

So are you saying that the speedo reads 73 when you are actually doing 80 - surely this is the wrong way round? 

Laney, your right, I got it the wrong way round, speed reads 80 and Sat Nav- Road Angel 73 mph 

I was getting concerned that I wouldn't be able to open my front door when I get home tonight because of the mountain of speeding notices on my door mat  

At least it builds a bit of safety net in I suppose - a poor show from VW though IMO  

[Cass]

More info/hack here:

http://forums.vwvortex.com/zerothread?id=2886728

Cass

[Pando]

At least it builds a bit of safety net in I suppose - a poor show from VW though IMO  
[/quote]

  Do VW actually make the speedoes, i doubt it.
Not exactly a major life affecting issue your speedo being a few mph out, as long as it is on the low side

[laney]

There's also a trick to show your speed on the climate temp display (I suspect people already know this).

I tried this on my way home from work last night - 70 on the speedo equated to 63 on the temp display 

Does anyone know if the temp display is likely to be more accurate?  Whats the difference between how the speeds are measured?

[illyun]

I tried this on my way home from work last night - 70 on the speedo equated to 63 on the temp display 

 

That sounds about right 

[Teutonic_Tamer]

Does anyone know if the temp display is likely to be more accurate?  Whats the difference between how the speeds are measured?

My educated guess would hedge towards the climate display being more accurate, simply because the speedo is an electro/mechanical component.

They both get their speed "info" from the same source, namely the ABS wheel speed sensors.  The signal is then "converted" into an mph/kmh figure for display on the climate and speedo by one of the cars ECUs.  That is why the raw data can be adapted on the VAG-COM, or at the dealer.

EDIT corrected my numpty error: By law, speedos can never under-read, but can over-read by up to 10%, so a genuine 70mph could be displayed on the speedo anywhere between 70-77mph.

My S4 seems to be very accurate, at an indcated 80, using cruise on arrow straight roads, GPS displays 79! 

[acwats]

Does anyone know if the temp display is likely to be more accurate?  Whats the difference between how the speeds are measured?

My educated guess would hedge towards the climate display being more accurate, simply because the speedo is an electro/mechanical component.

They both get their speed "info" from the same source, namely the ABS wheel speed sensors.  The signal is then "converted" into an mph/kmh figure for display on the climate and speedo by one of the cars ECUs.  That is why the raw data can be adapted on the VAG-COM, or at the dealer.

By law, speedos can never under-read, but can over-read by up to 10%, so a genuine 70mph could be displayed on the speedo anywhere between 63-70mph.

My S4 seems to be very accurate, at an indcated 80, using cruise on arrow straight roads, GPS displays 79! 

Sean, example given is the wrong way round - a genuine 70 must be shown on the speedo as 70+ (up to 77). This means when the speedo is showing 70 you are doing less. As you said the speedo cannot under read. e.g. actual 65 speedo 70

[Phil mcavity]

i thought with sat navs, they are not the most accurate due to satellite delay, as in all things to do with satellite due to time taken to send signal from 35000 kilometres away!

[illyun]

i thought with sat navs, they are not the most accurate due to satellite delay, as in all things to do with satellite due to time taken to send signal from 35000 kilometres away!

Phil, the signal travels at the speed of light and therefore takes about a tenth of a second to go 35k km...Also the delay is constant so you are simply looking at the speed as it was measured a split-second ago...  The kind of errors people are seeing with their Mk5 speedo's are far too large to be due to satellite delay.

[Teutonic_Tamer]

Does anyone know if the temp display is likely to be more accurate?  Whats the difference between how the speeds are measured?

My educated guess would hedge towards the climate display being more accurate, simply because the speedo is an electro/mechanical component.

They both get their speed "info" from the same source, namely the ABS wheel speed sensors.  The signal is then "converted" into an mph/kmh figure for display on the climate and speedo by one of the cars ECUs.  That is why the raw data can be adapted on the VAG-COM, or at the dealer.

By law, speedos can never under-read, but can over-read by up to 10%, so a genuine 70mph could be displayed on the speedo anywhere between 63-70mph.

My S4 seems to be very accurate, at an indcated 80, using cruise on arrow straight roads, GPS displays 79! 

Sean, example given is the wrong way round - a genuine 70 must be shown on the speedo as 70+ (up to 77). This means when the speedo is showing 70 you are doing less. As you said the speedo cannot under read. e.g. actual 65 speedo 70

Whoops, senior moment - Doh  .  You're exactly right.