Found out with our new Superb on 17" wheels that the speedo is showing some 5% more than actual speed. Actual meassured with four different GPS devices. Coming from Hyundai and Kia with much more accurste speedos I find that a bit too much for my liking, but 2% would be acceptable.

Doing a quick google search it looks like this is a standard VAG practice and it should affect only the speedo, but odometer is supposed to be accurate. There is also some mention of modifying some settings via VCSD to "fix" this issue.

Anyone done that change on Superb III or can confirm that this is possible to change?

Construction and Use Regulations demand a speedo can never under-read but can over-read by up to 10%. As the reading will be affected by everything from wheel size to tyre wear (your speedo reads differently at 8mm and 2mm tyre tread), there has to be a balancing act. You can tweak the speedo all you like, but as soon as you drive the thing it's already slipping in accuracy. If accuracy is important to you (can't think why especially, on a public road) then just use GPS as you said.

On summer tyres , 18" with approx 6mm tread all round my SII speedo reads +3 mph pretty much across the (legal) speed range, as checked on a recent trip to Scotland with my brother in the passenger seat checking on a couple of GPs units at 30, 50, 60 and 70 mph indicated. This suggests that it is an electronic add on, as if it were mechanical it would be different at each of the above speeds. I would guess the SIII would be similar

Edited 1 August, 20169 yr by Bristolf2b

My Octy on 16" reads around 5% over too

I would see it a slight advantage in that you have some margin of error for speed cameras!

Every car I've ever driven has over read by about 10% against GPS systems. That's a good thing IMO. 

Really didn't want to start the discussion why I prefer to have more accurate speedometer reading. It's not some big issue, but I know it can be more accurate, and from what I read doing some google search it seems quite possible that this can be achieved also on Superb.

So again, does anybody know if this is actually possible to change with VCSD?

On summer tyres , 18" with approx 6mm tread all round my SII speedo reads +3 mph pretty much across the (legal) speed range, as checked on a recent trip to Scotland with my brother in the passenger seat checking on a couple of GPs units at 30, 50, 60 and 70 mph indicated. This suggests that it is an electronic add on, as if it were mechanical it would be different at each of the above speeds. I would guess the SIII would be similar

This would indeed mean it's an electronical safety margin. (wheelsize etc would give it an x percentage difference). I wonder what would happen if you drive really slow, would there still be the same difference?

Or would it be possible to identify the cut-off point?

Thing is, in this modern age of just about every car having built in GPS, or some form of connectivity from a smart device, then why can't speedos run off the GPS? Obviously there would need to be a backup system for when there is no signal (such as the brief times traversing tunnels etc), but that could still default to the wheel speed sensors (as they currently do).

 

But, yes, it's an historical thing, speedos have never been accurate (even the police have to have them calibrated on a regular basis), hence the over reading. This also means that when you have an indicated 60mph on the speedo, you are actually doing 56mph, which is conveniently 1) the most fuel efficient speed & 2) the speed limit across most of America. Personally, over reading by 5% doesn't bother me, as I would rather it read too fast than too slow.