The majority of cars have a speed that has an indicated speed that is greater then the actual vehicle speed to comply with UNECE 39/00 (Rev 1) which is adopted by many countries including Australia with ADR 18/03. Volkswagen vehicles from around 2005 onwards can have this indicated speed adjusted to more accurately reflect the true speed by adjusting the deliberate error that is introduced in the instrument cluster.
Testing showed that my MY08 Golf GTI was over-reading by around 10%, the following table shows the GPS speed (handheld), the CANBUS speed which is what the odometer uses, the Indicated speed prior to the coding change and the corrected speed after the change.
The distance impluse number on my vehicle was set at 3, I changed this to the maximum value of 7 which was spot on what was required. You will require a VCDS./VAGCOM cable to make this change, the dealer could make the change but I understand that they will not do this.
Adjusting the distance impulse number does not alter the odometer, the distance impulse number is an error that is introduced for the speedometer needle.
Speedometer readings before and after correction
|GPS Speed||CANBUS Speed||Indicated Speed||Corrected Speed|
There are a number of ways to display the CANBUS speed, the easiest method is to use the full sized MFD and go to the speed alert screen, press ok until three dashes show and clock ok, the warning speed is your current CANBUS speed. This is a static display so use cruise control on a flat level surface.
The other way is to use the climate control system to display the speed. See the post on this site under reference for more information on this.
Volkswagen have some TSB’s out for this “problem”.
2005 – 2006 A5 Jetta and A5 GTI, Speedometer Display
If you receive a customer complaint that the speedometer displays a vehicle speed that is slightly faster than the actual speed, this is normal. The speedometer is operating per manufacturer specifications. Volkswagen does not manufacture vehicles where speedometers read slower than actual speed. To allow for possible differences in overall tire diameter with different tire manufacturers and wheel sizes, a factor is designed into the speedometer function that increases the displayed speed. This factor does not affect odometer readings, which show actual distance traveled. Do NOT replace ANY parts for this condition.”