Incorrect Speedometer - Golf mkVII?

[DV52]

I'm not sure if others have (or if they have noticed) this problem but I recently had need to take my Golf on a long trip to a neighbouring State. Now I'm not the kind of guy that likes to "fang" their car, but I do like to drive at the speed limit. So, I set cruise control on the Golf to 110 klms/hr and off I went down the mighty Hume highway.

At first I thought that the highway was full of cars breaking the speed limit because almost without exception, I was being passed by others (I'm old enough not to be troubled about others overtaking my car). But then I went under one of those bridges that displayed your speed and the reading said that I was travelling at 104 klms/hr. The digital reading on the car (which matched the analogue dial and the cruise control reading) said that my speed was 110 kms/hr. Yes, I know that it's better for legal and for fiscal reasons that the car's speedo reading be high, but this is an error of about 5.5%.


Has anyone else experienced this magnitude error in their speedo reading? What is a reasonable speedo error for today 's modern cars?

My expectation is that if I take my problem to a dealer, I'll get the usual VWA response which is "you should discuss your problem with someone who cares because we do not!" So, My other question is - can the speedo reading on a Golf be tweaked (perhaps with a VCDS cable)?

[Dutch77]

That sounds about right from my knowledge of the Mk6.

I believe more accurate data was available from the car - I saw a Polar FIS in action and IIRC these would display actual speed v what appeared on the dash/dials. I don't know if there is a Mk7 compatible version yet given the newer MFD displays but checking with the group buy details on here might shed more light.

Warning: if you like looking at data then a Polar FIS may not be wise.

[team_v]

ADR Design rules allow a speedo over reading error which will account for tyre wear and slight lapses in speed awareness from drivers.

People who claim they were caught for only doing a couple of km/h over the speed limit on the highway are usually doing about 9-10km over the limit on their actual speedo.

You can correct for this by getting slightly different tyre sizes so long as they are still legal and it will reduce the difference down to about 2%.


There was a huge discussion in the mk6 section about it and vcds but i don't think the tweaking of multipliers actually made any difference to the speedometer.

[DV52]

That sounds about right from my knowledge of the Mk6.

I believe more accurate data was available from the car - I saw a Polar FIS in action and IIRC these would display actual speed v what appeared on the dash/dials. I don't know if there is a Mk7 compatible version yet given the newer MFD displays but checking with the group buy details on here might shed more light.

Warning: if you like looking at data then a Polar FIS may not be wise.

Dutch77: ok, so you are saying that this magnitude error is not unacceptable? I don't know why, but I'm surprised. The fact that all those other cars kept passing me suggests that their speedos were more accurate (or that they all had a "lead-foot").

Thanks for the advice about the FIS. I had a quick look at this some time ago. It seems to be an interesting bit-of-kit. Fortunately for my bank account, I got the impression that the manufacturer hadn't yet upgraded the device to be compatible with the mkVII chassis. I'll have a dilemma when this happens!

[hoi polloi]

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Dupe post

[hoi polloi]

Yes, refer this ADR

http://rvcs-prodweb.dot.gov.au/files/ADR%201803.pdf

It's on the last page.. Passenger vehicles are Category M so the difference must be within 10% +6km/h but the speedometer can't under-read (if that makes sense)..

Test speeds for certification are 40, 80 & 120km/h

So as I see it -

At true 40km/h speedo reads between 40 & 50km/h inclusive
At true 80km/h speedo reads between 80 & 94km/h inclusive
At true 120km/h speedo reads between 120 & 138km/h inclusive

[Dutch77]

Dutch77: ok, so you are saying that this magnitude error is not unacceptable? I don't know why, but I'm surprised. The fact that all those other cars kept passing me suggests that their speedos were more accurate (or that they all had a "lead-foot").

In my opinion yes its fine and not dissimilar to industry averages.

The ADR is a mind-numbing 15 pages on the topic, but in simple terms: since 2006 it must not show less than actual speed, and not more than 10% + 4km/h.

A lot of drivers will either be factoring in this estimated variance, guessing as to how far over they can be before being picked up by camera (best not to experiment in Victoria), or indeed lead-foots.

[DV52]

Yes, refer this ADR

http://rvcs-prodweb.dot.gov.au/files/ADR%201803.pdf

It's on the last page.. Passenger vehicles are Category M so the difference must be within 10% +6km/h but the speedometer can't under-read (if that makes sense)..

Test speeds for certification are 40, 80 & 120km/h

So as I see it -

At true 40km/h speedo reads between 40 & 50km/h inclusive
At true 80km/h speedo reads 80 & 94km/h inclusive
At true 120km/h speedo reads 120 & 138km/h inclusive

hoi polloi; that's astonishing (and appalling)! That means (I think) that if I set my cruise control to 100klms/hr, the car can be travelling as slow as 84klms/hr. I know that as a society we have all become more conservative, but this is too much!

[Dutch77]

hoi pollo I; that's astonishing (and appalling)! That means (I think) that if I set my cruise control to 100klms/hr, the car can be travelling as slow as 84klms/hr. I know that as a society we have all become more conservative, but this is too much!

Still preferable to the old ADR that permitted a reading of +/- 10%.. especially when courts won't except the under-read as a defence.

[DV52]

Still preferable to the old ADR that permitted a reading of +/- 10%.. especially when courts won't except the under-read as a defence.

Dutch77: yes, that's true. But in the 21st century, I would have thought that we could rely on better metrics in our cars.

if I could press you for a further response (please), you hinted in a previous response that the FIS might be able to display true speed. How does it do this?