maybe someone can help, maybe its a ridiculous question, but if i dont ask, i will never know.

Is it possible to calibrate the speedometer with vcds? mine reads about 4mph below my actual road speed, and if possible, i would like it to be accurate.

I am not attempting to adjust the mileage, just the speedo so will still be within the bounds of legality.

I am aware that most vehicles speedo readings are only a rough guide so as to prevent unwanted speeding tickets etc, but i still would like to try.

Thanks in advance to anyone who can help.

I can't answer your question but it's odd that it under reads. Speedos always over read. Have you fitted larger diameter wheels? How do you know that it under reads, are you comparing it to GPS?

Ah no, my mistake. It reads 34mph when i am actually doing 30.

Pretty much standard, they all do it to about the same extent. At 80mph on the speedo the difference is usually greater with the true speed around 70mph.

but can it be calibrated?

You can fit a fully calibrated speedo (like the police do to their traffic cars) but i dont think you can adjust yours.

It obviously over reads a little so people don't complain they're getting done for speeding!

Not that I am aware, since things like tyre wear and pressures also have an impact on the speedo accuracy. Also GPS is not accurate either, so if you are using a Tomtom or garmin to obtain your speed then these are also out as well. A tomtom only periodically checks its GPS position, it then needs to perform calculations to get the speed. GPS positioning accuracy is not absolute, it can be several meters or more out which is why satnav systems use an average approximation to calculate speed.

It can be adjusted.......

http://www.myturbodiesel.com/1000q/a5/speedometer-read-fast-adjustment-VW.htm

As has been said, it's very common for a speedo to over read.

I looked into adjusting this on my Octavia.......It can be adjusted with VCDS, however the value you adjust (distance impulse number) also adjusts the odometer.

So if you correct a 10% over read on the speedo, the odometer will then under record mileage........apparently the odometer tends to be correctly calibrated despite the speedo over reading.

Because of this, I didn't bother with the adjustment.

It would be possible to modify the lookup table for the speedo in the cluster EEPROM (using a tool such as VAGdashCOM), but to be honest, it's just not worth it. If a new tyre has 10mm of tread and you run it down to 2mm, you are taking 16mm off the diameter.