Speedo Inaccurate

[Topend]

Hi,

Had our first long drive yesterday in my wife's new Golf VI. I fitted my GPS to check the speedo accuracy. It turns out the speedo is out by -7%, ie at an indicated 100km/h we are only traveling at 93km/h.

Does anyone know if the VW service guys would be able to correct this?

Cheers,
Dave.

[gregozedobe]

Does anyone know if the VW service guys would be able to correct this?

Cheers,
Dave.

Welcome aboard Dave. Lots of useful info here.

The short answer is "No". Some of the more switched on techs probably could, but I doubt that any of them would be willing to do it for you. This under-reading error is common on many modern cars due to ADR requirements.

If you want lots more info try the search facility, as there have been many threads covering this issue and discussing what options are available.

[The_Hawk]

This under-reading error is common on many modern cars due to ADR requirements.

There is no good reason for this, the ADR says you CANT read under, but you can read over by up to 10% (7% in your case). If you run the numbers of all the standard wheel and tyre combo's there are available (and I have for the Mk5) there is about 1% difference in rolling diameter across the range. So even if you set the speed right for the largest wheel, it should only be out by 1% for the smallest.
Add to this that tyre wear will only ever make you go slower (as the tyre wears it gets a smaller rolling diameter) and even then it's only about 4%.

Why all the speedo's are so wrong is beyond me, especially when there are so many reports of people bringing up the speed on the digital readout and it's much closer to reality :S

[Topend]

Thanks for the responses. I should add that my speedo and MFD don't read the same either. There is a 2 or 3 km/h difference.

It is annoying that in order to travel at 100km/h the speedo has to indicate 108km/h and for 110km/h the speedo has to indicate 119km/h. We have 130km/h limits up here (use to be unlimited) so it will be interesting to see what the speedo indicates at that speed.

Cheers,
Dave.

[brad]

It will pretty much be 7% all the way through.

On my Octavia, I have to do 120kmh to maintain a true 110kmh. It must be an endemic thing among the car manufacturers because I'm constantly having to flash cars on the M5 to get out of my way & they think I'm being aggro - all I want to do is get to work and stick to the speed limit.

[Coaster]

Thanks for the responses. I should add that my speedo and MFD don't read the same either. There is a 2 or 3 km/h difference.

Is the MFD more accurate than the analogue (speedo) display?

[Topend]

Is the MFD more accurate than the analogue (speedo) display?

Yes, 2km/h closer to the true speed.

[Corey_R]

I've also tried tweaking the Distance Impulse Number because on the R with 19s I'm doing 94-95kmh when the needle and digital speedo are showing 100kmh. I want to know exactly what I am doing when I look down, not some mathematical acrobatics. The best I could do was set it to Distance Impulse Number 8 and the country to Australia and now I'm only about 3kmh out.

Keep in mind now that you've done that, that your entire trip meter is now wrong, including fuel consumption and Odometer. The car KNOWS that it's doing 93/94km/h, but tells you that you're doing 100km/h because it's required to do that by law. If you were to do 100km/h and then reset your trip meter, it would then tell you CORRECTLY that you're averaging 93/94km/h. Now that you've changed those figures, it'll give you incorrect information, incorrect fuel consumption, incorrect range til empty, etc etc.

[Fab_R]

Keep in mind now that you've done that, that your entire trip meter is now wrong, including fuel consumption and Odometer. The car KNOWS that it's doing 93/94km/h, but tells you that you're doing 100km/h because it's required to do that by law. If you were to do 100km/h and then reset your trip meter, it would then tell you CORRECTLY that you're averaging 93/94km/h. Now that you've changed those figures, it'll give you incorrect information, incorrect fuel consumption, incorrect range til empty, etc etc.

Good info Corey...

Stevie, I think you may want to change it back man?

[SteveK]

Keep in mind now that you've done that, that your entire trip meter is now wrong, including fuel consumption and Odometer. The car KNOWS that it's doing 93/94km/h, but tells you that you're doing 100km/h because it's required to do that by law. If you were to do 100km/h and then reset your trip meter, it would then tell you CORRECTLY that you're averaging 93/94km/h. Now that you've changed those figures, it'll give you incorrect information, incorrect fuel consumption, incorrect range til empty, etc etc.

Are you sure about that Corey? Not having a go, just trying to understand your thinking. I've worked with guys to do tunes on various local cars and adjusted speedos via electronics to correct deficiencies and my understand is that you are just adjusting the reading from the rolling tyre to the speedo. From there the CPU will use that data to correctly calculate average speeds and fuel usage etc. It's all based on data lines and if the speedo data is incorrect, from the wheels, not what is displayed, then YES the rest of your data is wrong. When compared to various other GPS based speedometers the car is definitely out and after the correction it is closer. I still don't see how that is bad?

Cheers,
Steve