Incorrect Speedometer - Golf mkVII?

[CT69]

Anthony et al: is it just me, or does anyone else find all this strange. What's being said is that the car actually knows the correct distance that it travels and the car also knows correct time. But for some odd reason, the car can't/won't tell the driver the correct speed. Doesn't this seem just a little bit absurd? Why would the good burghers in Germany take these correct metrics and manipulate the speedo reading to give the wrong number?

I think it does know the correct speed (hoi polloi's method implies the speed average display is accurate).
The makers presume we are not adult enough to be trusted with the information of our actual speed!
(and you can kind of see why....)

[giveway]

The car knows the correct speed and records the odometer correctly. The display is deliberately incorrect and reads over what speed your actually doing. If you plug in a OBDII adapter and get the speed reading off that it almost matches identical the GPS speed. Usually +-1 kph.

I have a Android app called Torque Pro and a Bluetooth OBDII dongle and Bluetooth 10hz GPS dongle. Both my BMW and VW polo are out by +10% on the dash compared to both the GPS and OBDII speed readings.

When I get my new MK7 GTI PP on the weekend I'll see what it says and try and get a screen shot to show you guys.

Sent from my Nexus 7 using Tapatalk

[brad]

I recall a service bulletin from Volkswagen Group Australia that refutes this, as it is measured based on the car's "true" speed, not the "corrected" speed displayed on the speedo.

I checked it against several of the 5 x 1km check distance signs on various motorways & my odometer would consistantly click over about 1 swimming pool lengths at marker 1; 2 at marker 2 and would be on about 5.2km at marker 5 and click over to 5.3 just after passing the marker

250m in 5km is approximately 750km every 15,000km. By the time you get to my mileage you've been ripped off almost 7,000km if you haven't made some adjustments.

VW can refute all they want but they've refuted plenty of other stuff that has proven to be a fantasy in their own minds.

[DV52]

I think it does know the correct speed (hoi polloi's method implies the speed average display is accurate).
The makers presume we are not adult enough to be trusted with the information of our actual speed!
(and you can kind of see why....)

CT69: Yes, I suspect that you are correct. The design philosophy that you are describing (i.e. reversion to the lowest common denominator) is often employed on lots of stuff these days. Personally, I find this mentality extremely patronising. Why can't the designers at VW simply assume that as adults, we are indeed accountable for whatever decision we make about the speed that we travel? Anyhow, time to get off my soapbox!

[brad]

CT69: Yes, I suspect that you are correct. The design philosophy that you are describing (i.e. reversion to the lowest common denominator) is often employed on lots of stuff these days. Personally, I find this mentality extremely patronising. Why can't the designers at VW simply assume that as adults, we are indeed accountable for whatever decision we make about the speed that we travel? Anyhow, time to get off my soapbox!

You just have to look at the number of people that try & transfer blame to anyone but themselves whenever something goes wrong.
eg: Irish tourist falls into Darling Harbour after exiting nightclub 10 metres away. Irish tourist sinks like a stone & drowns. Immediately there are calls that City of Sydney Council / Darling Harbour Authority should surround the whole of darling harbour with a fence. I feel sorry for the guy because all the holes in the cheese lined up at once but maybe there was something he could have done himself to prevent his early demise.

I'm pretty lucky & get to drive a lot of modern cars. All of them have a speedo that reads under - this is a good thing for innatentive drivers or those that think the speed signs are an instruction to proceed at that pace no matter what rather than an advisory maximum. Without a doubt the VW group cars are the worst by a factor of 2 and this is far too conservative.

[Idle]

Anthony et al: is it just me, or does anyone else find all this strange. What's being said is that the car actually knows the correct distance that it travels and the car also knows correct time. But for some odd reason, the car can't/won't tell the driver the correct speed. Doesn't this seem just a little bit absurd? Why would the good burghers in Germany take these correct metrics and manipulate the speedo reading to give the wrong number?

I'm given to believe that it's because their law requires it.

Probably another example of woolly-headed thinking on high.

[i286]

Speedo's are designed to over read since I can remember. That is the case across all makes all around the globe. I don't see problem with that at all. Some makes over read more some less, but generally they are all doing it as 7 kmh or less. My new Highline over read by 5kmh. When driving in 110kmh zone I keep speedo at 115kmh and that's it.
Do not assume that advisory speed check points are accurate, many of them are not, if working at all in the first place. Also, using cruise control does not mean that your money and points are safe, at some points during your journey you will go over the speed limit even if you set the cruise correctly.

Why are they designed in that way? Obviously there are design rules in every country and manufacturers have to go by them if they want to sell cars there. Some of the other reasons are, in the case of our golf, but that is the case with every other make as well, that there is five different wheel size options across the range. Would they make different gear and calibrations for 90tsi or Comfortline or Highline?
When tyres are new they usually have 8mm thread depth. If people are replacing them when it is only 2mm left that means that overall diameter is reduced by 12mm. Then there is load which affects rolling diameter, then incorrect tyre pressure.

Only gps would give 99.something % accurate speed irrelevant of the wheel size or the load or the inflation pressure.

[singlespeed]

Normal 7% speedo error. Using a GPS you can verify it. Also, using a ScanGauge I can see the OBDII knows the true speed of the car.

[DV52]

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

Brad: Did some more reading as you suggested (thanks for the link). Seems that the culprit is indeed the "distance impulse number" in the Instrument module controller (VCDS speak). As I understand, this number alters that speedo reading without changing the odometer reading (interesting point was made about the legal reason for the odometer to read correctly, else VW could be sued for not providing the correct warranty kms).

Anyhow, only one problem with what the Ross-tech guys are saying about changing the "distance impulse number" to get a more accurate speedo reading. Can't find any such number in the byte/bit structure for the instument controller on my mkVII. The only thing that comes remotely close is something called "Tire Circumference" see bit 0-2 below




Anyone know what this is?

[Idle]

It's the distance the car travels per wheel revolution — might have something to do with speedo calibration.

For instance, if it could be increased slightly the reading ought to be closer to correct.

Then again, it may have nothing to do with the speedo at all, or only apply to the odometer.

Rosstech might be able to elucidate.