I would think that no spedometer can be perfect. Your tires are getting smaller every day. Every day they wear a little more. That would make the tires rotate faster. That is why on all wheel drive cars they want you to replace all four when you need new ones. I know the police department here has their spedometers calibrated often.

 

When I'm in town, I drive at the flow of traffic or slightly faster. When I'm away from other cars, I drive as fast as I think I can get away with... the speedometer is just a curiosity. Who cares if it's a few off?

 

Yep, it's intentional. Every German car I've owned - I lump MINI/BMW into that - has speedo that displays up to 10% higher than actual speed (by law can't display lower than actual). Many posts already about it here.