Speedometer Accuracy

[--Doug--]

Quick question; my Kodiaq speedometer under-reads by 4 mph at 70 mph (indicated) as checked by GPS signal on my iPhone. So -5.7%. I had to set 74 mph (indicated) speed to achieve a GPS speed of 70 mph. This seems excessive to me because the figures on my previous Octavia were 68 and 72 mph.

 

The car feels slower at a given indicated speed to me and my son also commented on this but I dismissed it because the car is taller and is thus further from the road so the difference is actually perceivable.

 

Q1. Are these figures typical?

 

Q2. Can a correction be applied using VCDS coding?

 

Another minor irritation is that the drive mode selection reverts to Normal after ignition switch off but I’m sure it stayed on the previously selected mode on my Octavia. Again, does anybody know if this can be changed by VCDS coding?

 

Thanks in anticipation.

 

Doug

Edited 1 June, 2019 by --Doug--

[AndyMorris]

It's done on purpose so we don't get nicked for speeding! & yes my kodi is the same. 

Can't help with the coding question.... but personally the two things mentioned don't bother me. 

 

 

[--Doug--]

2 hours ago, AndyMorris said:

It's done on purpose so we don't get nicked for speeding! & yes my kodi is the same. 

 

Fair enough Andy and thanks for your reply. These issues bother me to a small degree because they’re different to my previous car with no apparent reason why.

 

At work, I respond to alarms and indications which are set at prescribed levels with an appropriate tolerance band and I expect to do the same when driving. +/- 2 mph is reasonable in my judgement, but +/- 4 mph is not. Simple as that.

 

If people stick to the indicated speed and the indicated speed is accurate they shouldn’t get nicked for speeding. It is wholly illogical to set it to overread by nearly 6% IMHO. That would be as daft as Rolex setting all their watches to run 5 minutes fast so their owners are never late for a meeting.

 

Doug

[Kenny R]

Tyre size, tyre wear and probably pressure have an effect on the speedo accuracy. I find the 235x50x19 under read by 2-3 mph @ 70 mph but when I have my winter wheels on 215x65x17 the speedo is almost totally accurate.

On my 2018 SEL I have individual mode selected with the sports steering selected and it stays on this setting

Edited 1 June, 2019 by Kenny R

[--Doug--]

1 hour ago, Kenny R said:

Tyre size, tyre wear and probably pressure have an effect on the speedo accuracy. I find the 235x50x19 under read by 2-3 mph @ 70 mph but when I have my winter wheels on 215x65x17 the speedo is almost totally accurate.

On my 2018 SEL I have individual mode selected with the sports steering selected and it stays on this setting

Thanks Kenny.

 

These are the brand new manufacturer fitted Continental tyres which I inflated to 38 psi all round two weeks ago. I'll re-check the manual regarding the drive mode selection to see if I can work out why it isn't latching.

 

Out of interest, I used

 

https://www.tyresizecalculator.com/tyre-wheel-calculators/tire-size-calculator-tire-dimensions

 

to check the difference in your case and 

 

235/50/R19 tyres have a diameter of 718 mm and rotate 715 revs/mile

215/65/R17 tyres have a diameter of 711 mm and rotate 722 revs/mile

 

So not a lot in it (about 1%). :-/

 

Doug

[Exkiwi]

Posters are saying their speedo is UNDER READING   ITS NOT    its OVER reading as required by law. Dont know about Europe but in Australia the ADR Rules  state  that a speedo  can over read by up to 10kph at 100kph + - 4% so legally it could read 114 and still be legal.   IT may NOT READ LESS THAN ACTUAL SPEED. Most are set at +4 to +7    My TIguan was  7 over at 100kph and by using a OBD  Eleven  I changed the wheel size to bring it bang on.  Legal Wheel and tyre sizes are on the plaque in your fuel  cap  so if you stick to them you will be OK

 

Changing drive modes is for gears and suspension settings and will have nothing to do with speedos

Edited 20 March by Exkiwi
Added to the info

[Routemaster1461]

in the UK the speedo can over-read by 10%, most seem to be about 5%. By law the speedo must not under-read at all.

[Avocet]

Strictly speaking, for a mass produced car, it can over-read by 10% + 2.5 MPH, (At all practical speeds), but cannot under-read at all.  Same in the EU.  Paragraph 5.4 of ECE Reg 39:

https://unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/2018/R039r2e.pdf

As a result, manufacturers always set them a few % high.

[linni]

Mine does + 4 kmh on 20" summer wheels and + 6 kmh on 18" winter wheels. But I drove a replacement Octavia last year which was deadly accurate.

[Avocet]

Unusual!  You would have thought that VAG would be on their best behaviour when it came to compliance,  - after the emissions scandal!

[Exkiwi]

Has nothing to do with any scandals and is a legal  requirement in most countries that speedos do that    Look it up in the regs if you dont believe us.

[Winston_Woof]

https://hansard.parliament.uk/lords/2001-03-12/debates/e08bd310-b9ce-4441-86ac-7e04348d8bdb/SpeedometerAccuracy
 

The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests.


 

[Exkiwi]

Virtually the same as Australia

[Avocet]

6 hours ago, Exkiwi said:

Has nothing to do with any scandals and is a legal  requirement in most countries that speedos do that    Look it up in the regs if you dont believe us.

 

The regs I just quoted, (and posted a link to) a couple of posts above this one, you mean?🤣

 

14 hours ago, Avocet said:

Strictly speaking, for a mass produced car, it can over-read by 10% + 2.5 MPH, (At all practical speeds), but cannot under-read at all.  Same in the EU.  Paragraph 5.4 of ECE Reg 39:

https://unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/2018/R039r2e.pdf

As a result, manufacturers always set them a few % high.

 

 

Yeah, I know.  (I type approve cars for a living)!

 

The bit that was unusual, was linni's replacement Octavia speedometer being "deadly accurate".  That's very rare.  If a manufacturer makes a speedometer "deadly accurate" he runs the risk that one of them ends up under-reading by a tiny amount (which is not permitted under Reg 39).  If the type approval authority that issued the approval catches him doing this, they can force a recall, withdraw his approval (so he can't register any more cars), and the European Commission can fine him up to €30,000 PER NON COMPLIANT CAR!

 

So...  When the type approval regulation tells you the speedo has to read anywhere between true speed and 10%+4 km/H ABOVE true speed, but under no circumstances ANY amount BELOW true speed...

 

...and you know the fines and penalties if they catch you having built a car outside of those limits...

 

What would YOU do, if you were a car manufacturer?😉

[Ootohere]

@linni  did not say if the replacement Octavia driven last year was on the OEM tyres as it left the factory.

 

I also drive cars that occasionally are 'Deadly accurate' as to the speedo and speee being done. They are not on the factory size tyres though. 

[Avocet]

2 hours ago, Winston_Woof said:

https://hansard.parliament.uk/lords/2001-03-12/debates/e08bd310-b9ce-4441-86ac-7e04348d8bdb/SpeedometerAccuracy
 

The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests.


 

 

Construction & Use is old UK-only legislation.  Unlike type approval (which applies to the car "as manufactured", the C&U regs apply to the car "as used".  In other words, if you buy a new car, the responsibility for speedometer accuracy rests with the manufacturer.  However, you can go out and buy it a different sized set of wheels and tyres and by doing so, make the speedo inaccurate.  Clearly, the manufacturer can't take responsibility for that, so it's down to you.  That's why we need two sets of similar regulations.

 

As you can see, C&U calls up the same ECE Reg 39 that I mentioned earlier. (It also gives the alternative option of complying with the old EC Directive, but that is no longer used these days by mainstream manufacturers and it's requirements for accuracy were exactly the same as in ECE Reg 39 anyway).

 

However, I don't recognise the last bit about a production tolerance of 5%+10 km/h?  That's definitely not true. It would be illegal for a start!  At a true speed of (say) 20 km/h, the car's speedometer would read 21 km/h (5% of 20) Plus another 10 km/H = 31km/h!  Whereas in fact, the type approval regulations would only allow a maximum of 10%+4 which would be 26 km/h at a true 20.

[Winston_Woof]

18 minutes ago, Avocet said:

 

Construction & Use is old UK-only legislation.  Unlike type approval (which applies to the car "as manufactured", the C&U regs apply to the car "as used".  In other words, if you buy a new car, the responsibility for speedometer accuracy rests with the manufacturer.  However, you can go out and buy it a different sized set of wheels and tyres and by doing so, make the speedo inaccurate.  Clearly, the manufacturer can't take responsibility for that, so it's down to you.  That's why we need two sets of similar regulations.

 

As you can see, C&U calls up the same ECE Reg 39 that I mentioned earlier. (It also gives the alternative option of complying with the old EC Directive, but that is no longer used these days by mainstream manufacturers and it's requirements for accuracy were exactly the same as in ECE Reg 39 anyway).

 

However, I don't recognise the last bit about a production tolerance of 5%+10 km/h?  That's definitely not true. It would be illegal for a start!  At a true speed of (say) 20 km/h, the car's speedometer would read 21 km/h (5% of 20) Plus another 10 km/H = 31km/h!  Whereas in fact, the type approval regulations would only allow a maximum of 10%+4 which would be 26 km/h at a true 20.

Aye am aware that is a UK reference.

Are you suggesting one of our MPs gave an incorrect answer in Parliament (Hansard is the official transcript of what's said there)  ;o)

Edited 28 March by Winston_Woof

[Paikesejanku]

It is possible to configure it using OBD11 or similar, to show accurate speed with your current wheels.

[Avocet]

I sure am!  It's pretty fishy that anything in Hansard (and thus UK-based) should talk in km/h for a start, but yeah, he's wrong.  (Not just any MP, either, a member of the House of Lords)!    The lowest speed that the ECE Reg 39 test is carried out at, is 40 km/h  even at that speed, the noble lord's claim that manufacturers apply a tolerance of 5% + 10km/h, would put the vehicle out of compliance for both type approval and C&U.  At a true 40 km/h it would display 52 km/h (which is above the maximum of 46.5 permitted by Reg 39).  Either he was wrong, or there's a typo in Hansard and he meant 5% + 1 km/h.  (Even then, he's wrong because all manufacturers don't apply the same tolerances in production.  They can apply whatever they like, so long s they stay within the legal requirements!

[Avocet]

26 minutes ago, Paikesejanku said:

It is possible to configure it using OBD11 or similar, to show accurate speed with your current wheels.

Speedometer signals aren't always linear (in fact, they're usually NOT perfectly linear), so you can probably only configure it to be exactly accurate at one particular speed.  (And of course, with one particular  tyre, at one particular temperature, pressure and state of wear).

[Winston_Woof]

I'll be honest, I normally use the GPS speed on Waze ;o)

[Paikesejanku]

According to my experience with My2023 Kodiaq, it aligns accurately with phones GPS speed measurement in Waze at least from 0- 120 km/h. Very linear for my car.

Edited 28 March by Paikesejanku

[Avocet]

1 minute ago, Paikesejanku said:

According to my experience with My2023 Kodiaq, it aligns accurately with phones GPS speed measurement in Waze at least from 0- 20 km/h. Very linear for my car.

 

20 is a pretty low speed though?  What is it like at (say) 100 km/h?

[Avocet]

4 minutes ago, Winston_Woof said:

I'll be honest, I normally use the GPS speed on Waze ;o)

Yes, GPS is always more accurate.  You might ask why manufacturers don't just use the GPS signal to power the speedometer?  That's because if they did, they wouldn't be able to take full "manufacturer responsibility" for their type approval.  Part of their speedometer "system" would be dependent on the American government (or Galileo or Glonass) and outside of their control.

[Paikesejanku]

4 minutes ago, Avocet said:

 

20 is a pretty low speed though?  What is it like at (say) 100 km/h?

Typo, I wanted to type 120. I agree that there are decimal place differences, but as i am not that accurate anyway - don't care.

Edited 28 March by Paikesejanku