My speedo reads 5 mph too high as well. My dealership told me it's common and that nothing can be done about it. The conspiracy theorist in me wonders if it's not at least in part an attempt to improve real world fuel economy numbers. Driving 65 instead of 70 is a sneaky way to get unknowing customers a bump in their mpg's.
2014 Touareg TDI R Line
2012 Q5 2.0T