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?

[CT69]

From my very simplistic point if view, I would just like to know exactly how fast I am driving; I had hoped that in this great age of technology, that might be possible!
I would accept that there would be an error of 1-2%.
Next question: if my speedometer reading is out by more than 5%, can I trust that my ODOMETER is not similarly inaccurate?
If VW or any vwwc subscribers know the answer to that I would be most interested (obvious ramifications for wear and tear, resale etc).

If the ODO is in fact accurate, then I could at least work out the amount of over-read on the speed (triggering lap timer and ODO-trip simultaneously, driving at a constant speed on the highway under cruise control for a while, then doing some simple maths). Not much point to this experiment if the ODO is buggered too.

Maybe I should check to see if the lap timer counts in actual seconds......

[b c]

I don't want to speed, but I also don't want to travel slower that I otherwise need to. I took a car GPS with me on a couple of trips and checked the MFD digital speedo against the independent GPS speedo (previously compared to roadside displayed speeds). Showed the digital over-reading by about 4kmh. Probably time I did it again.

[team_v]

From my very simplistic point if view, I would just like to know exactly how fast I am driving; I had hoped that in this great age of technology, that might be possible!
I would accept that there would be an error of 1-2%.
Next question: if my speedometer reading is out by more than 5%, can I trust that my ODOMETER is not similarly inaccurate?
If VW or any vwwc subscribers know the answer to that I would be most interested (obvious ramifications for wear and tear, resale etc).

If the ODO is in fact accurate, then I could at least work out the amount of over-read on the speed (triggering lap timer and ODO-trip simultaneously, driving at a constant speed on the highway under cruise control for a while, then doing some simple maths). Not much point to this experiment if the ODO is buggered too.

Maybe I should check to see if the lap timer counts in actual seconds......

The odometer uses the "uncorrected" speed from the engine/ecu so it will be correct, this was covered in detail in the mk6 section.

Again, a simple tyre size change can remedy this issue.

[brad]

My Skoda Octavia MK2 (like a MKV Golf) was 10% out ex-factory with 205/55r16 tyres.
225/45r17 brought it a bit closer (7%)
225/55r16 closer still (4%)
225/50r17 is about 2%- unfortunately it also exceeds the maximum increase in diameter, so is illegal. It's probably a bit too accurate for most people as teh width of the speedo needle is more than 2km

You haven't actually said what model MK7 you have or what size tyres you have but it is possible to go up a size & maintain the same profile & you'll make up a lot of the discrepancy.

The alternative is to buy a decent GPS & use the speedo function in that.

On some of the VWs/Skodas it is possible to tweak the speedo reading a bit using VCDS adjusting the distance impulse number.
Speedometer fast error adjustment and correction - mk5, mk6 | VW TDI forum, Audi, Porsche, and Chevy Cruze diesel forum

Check your odometer as well because that is also dramatically wrong - but not the full 10%. I think I worked out that 15,000km on the odo was only 14,300km actual, so you get services done earlier than needed & when you go to sell your car the recorded mileage is much greater than the actual milage. I guess the ideal situation would be to make the odo close to 1=1 & that will sort out the speedo with a bit of safety margin still available.

[DV52]

Again, a simple tyre size change can remedy this issue.

team_v: help me out here! Which way does it work? A larger tire results in a larger speedo reading - or the other way around?