Incorrect Speedometer - Golf mkVII?

[hoi polloi]

Aren't all speedos nowadays electronic? They can make the needle say what ever they want

Indeed they can.. At the end of it all they're just counting voltage pulses of one form or another - one pulse for a given unit of distance (or more accurately, angle of wheel rotation) the rate of pulses per unit of time giving speed..

Back to the ADR though, there is nothing that says the speedo *has* to be electronic - hence probably the very wide tolerance allowed not withstanding the fact the speedo can't under read... The ADR even specifies a temperature for testing (23ºC IIRC) which would affect mechanical speedos (with drag cups, hair springs etc.) more than electronic ones.. Practically every car that has an engine management computer needs a digital speed input from somewhere so it makes sense that speedos be digital too..

Personally, the only car I ever came across that was fuel injected with a mechanical speedo was the 3.0L VL Commodore (the one with the Nissan engine)... I'm sure there are others but when you consider the 5.0L V8 at the time was still carby fed you can kinda see why...

- Anthony

[CT69]

According to my Garmin my speedo shows 5km/h too much at 80,100,120 - so it doesn't stick to a % increase. It also indicates 100km after only doing a real 99km. All better than my Forester which is a consistent 7% out in the speedo but only 1.3% out on the odo.

Using GPS, my speedo overreads a consistent 5km/hr at all speeds, which is OK.
I was surprised to see the ODO under read, albeit by a tiny amount (Golf trip said 99.0km, GPS said 99.75km, less than 1% error)

[martinnw]

I had a similar issue with my Mk6 103 TDI. I checked the indicated speed against a Tom Tom GPS.
With the Mk6 I set the cruise to 125klm/h for 110 klm/hr speed limit & drove through the speed traps no problems over 3 years off hwy driving.
With the Mk7 103 TSI its a bit less & the difference is greater at higher speeds.
Cruise set to 67 for 60, 88 for 80, 110 for 100 & 123 for 110.

[Mountainman]

I had a similar issue with my Mk6 103 TDI. I checked the indicated speed against a Tom Tom GPS.
With the Mk6 I set the cruise to 125klm/h for 110 klm/hr speed limit & drove through the speed traps no problems over 3 years off hwy driving.
With the Mk7 103 TSI its a bit less & the difference is greater at higher speeds.
Cruise set to 67 for 60, 88 for 80, 110 for 100 & 123 for 110.

If I did that with mine I wouldn't have a license now. It is consistently 5km/h above the true speed. Indicating 125 is really 120 and in QLD we'd get booked for that in a 110 zone.

[brad]

If I did that with mine I wouldn't have a license now. It is consistently 5km/h above the true speed. Indicating 125 is really 120 and in QLD we'd get booked for that in a 110 zone.

That would be why you'd double check the true speed using at least one GPS rather than just taking blind advice from a forum.

All our cars are different & the diameter of our tyres can vary markedly from brand-to -brand even if they are the same nominal size.

Martinnw seems to be at the extreme end of innacuracy.

[team_v]

That would be why you'd double check the true speed using at least one GPS rather than just taking blind advice from a forum.

All our cars are different & the diameter of our tyres can vary markedly from brand-to -brand even if they are the same nominal size.

Martinnw seems to be at the extreme end of innacuracy.

I was going to say the same thing.
Seems like a 10% speedo overreading.

[ClownDog]

As for checking the speedo itself - if you don't have a GPS handy this works well at getting the uncorrected speed out of the car..

- Set the cruise control to the speed you want to check, say 100km/h
- With the vehicle at speed reset the trip computer
- Select the average speed readout
- Initially it will show dashes but after a few seconds the average speed is shown which is the uncorrected speed and actually quite close to GPS speed..

That sounded like a great idea until I actually tried it. I don't know if I'm missing something or maybe your mk6 is different to my mk7, but that doesn't work for me. Resetting the trip computer doesn't make any difference to the average speed display in my GTI - because it is displaying the average speed since start, not since resetting the trip computer. Hopefully I am missing something and someone can tell me how to display the average speed of a trip?

[Mountainman]

Hopefully I am missing something and someone can tell me how to display the average speed of a trip?

On the touchscreen, press car, go to settings (the cog wheels in the bottom right corner) scroll to multifunction display, scroll to second bottom option to reset "since start". I have never tried it while the car is in motion though.

[ClownDog]

On the touchscreen, press car, go to settings (the cog wheels in the bottom right corner) scroll to multifunction display, scroll to second bottom option to reset "since start". I have never tried it while the car is in motion though.

Ah, thanks! So I was missing something. I didn't realise the 'start' could be reset in any way other than taking the key out and restarting the car. I'll give it another go this weekend.

[Mountainman]

Ah, thanks! So I was missing something. I didn't realise the 'start' could be reset in any way other than taking the key out and restarting the car. I'll give it another go this weekend.

The "since start" has a 2 hour no drive delay before it resets itself, so taking the key out and inserting it again won't do anything.

[NCGR1]

....But in the 21st century, I would have thought that we could rely on better metrics in our cars.....

All manufactured devices have tolerances. In the case of the speedo in a car, there are a lot of items that all have a tolerance to be considered. No car can leave the factory indicating lower than actual. Certainly calibrating each and every vehicle at the end of the line would not be practical.

I really don't see the problem here. You know that the indicated is a bit high from actual. Make the adjustment in your hear if you just have to be traveling exactly the speed limit.

[DV52]

All manufactured devices have tolerances. In the case of the speedo in a car, there are a lot of items that all have a tolerance to be considered. No car can leave the factory indicating lower than actual. Certainly calibrating each and every vehicle at the end of the line would not be practical.

I really don't see the problem here. You know that the indicated is a bit high from actual. Make the adjustment in your hear if you just have to be traveling exactly the speed limit.

NCGR1: thanks for the advice. I'm constantly having to make the mental calculation that you suggested to correct for the inherent error in my speedo. In fact, I'm quite proud of how good I've become at doing this because I can now do the calculation without counting using my fingers - very handy because I no longer need to take my hands off the steering wheel! (Just kidding)

My point (which i didn't make clearly enough) is that the process that is being used for speedo calibration on modern cars is a "nanny" protocol. Yes it's being driven by local legislation like the ADR and by the threat of litigation that seems to be rife everywhere.

IMO, a far better approach would be to leave the responsibility for keeping to the speed limit entirely on the driver (and to have the speedo read accurately). I suggest this because ultimately there can be only one person who can be accountable for this function.

For me, the one vital question on this matter is this: why should the driver be forced to make the mental calculation that you suggest to figure out how fast the car is really going? Isn't this the sole purpose of the speedo?

And then there is this other question: if it is really necessary to have speedos in Australia under-read by an amount for safe driving, then why not simply change the speed limit to this quantity? Why have the speed limit sign say one thing- and then force car manufacturers to set their speedos to say another thing? Doesn't make sense to me.

Anyhow, I'll now get off my soapbox. Thanks for listening




If you look at the rich functionality of the options that

[Paul_R]

If anyone isn't happy with their speedo readout you can change it. I noticed the other day a setting in the menu for winter tyres that allows you to adjust the readout (Mk 6 R). Haven't fiddled with it. I just set cruise 5 above posted limit, 10 in 110 zones, and haven't had a ticket yet. From memory GPS and overhead gantry speed checks give 113 with cruise set at 120. Might be 1 or 2 higher.

[Guest001]

NCGR1: thanks for the advice. I'm constantly having to make the mental calculation that you suggested to correct for the inherent error in my speedo. In fact, I'm quite proud of how good I've become at doing this because I can now do the calculation without counting using my fingers - very handy because I no longer need to take my hands off the steering wheel! (Just kidding)

My point (which i didn't make clearly enough) is that the process that is being used for speedo calibration on modern cars is a "nanny" protocol. Yes it's being driven by local legislation like the ADR and by the threat of litigation that seems to be rife everywhere.

IMO, a far better approach would be to leave the responsibility for keeping to the speed limit entirely on the driver (and to have the speedo read accurately). I suggest this because ultimately there can be only one person who can be accountable for this function.

For me, the one vital question on this matter is this: why should the driver be forced to make the mental calculation that you suggest to figure out how fast the car is really going? Isn't this the sole purpose of the speedo?

And then there is this other question: if it is really necessary to have speedos in Australia under-read by an amount for safe driving, then why not simply change the speed limit to this quantity? Why have the speed limit sign say one thing- and then force car manufacturers to set their speedos to say another thing? Doesn't make sense to me.

Anyhow, I'll now get off my soapbox. Thanks for listening
If you look at the rich functionality of the options that

The thing is that 99% of the driving population wont even know about this as witnessed by the number of cars you can pass who think they are doing 100kph and are on the speedo but not actual.
Why not just drive to the speedo and save yourself the mental worry of working out the difference.

Almost everyone else is and you wont get there much earlier anyway.
If you are so worried get a VCDS cable and adjust the settings. Is apparently able to be done to get it more accurate.
The reason the rule is there is to allow for tyre wear and different size tyres fitted to model variants.
However VW are particularly bad, both of mine are 7 k out at 100kph.
In contrast my Toyota Landcruiser was bang on at 100kph with standard tyres.

[Mountainman]

The thing is that 99% of the driving population wont even know about this as witnessed by the number of cars you can pass who think they are doing 100kph and are on the speedo but not actual.
Why not just drive to the speedo and save yourself the mental worry of working out the difference.

What I have noticed is that most people are aware of the speedo inaccuracy and seem to sit around an indicated 105/110 in 100 zones yet soon as they come to a fixed speed camera they all drop back to an indicated 100 or less. So much for having confidence in their speedo readings.