I think you'll find this is the case for most speedo's when compared to Satnav's. My guess is that this gives the car manufactures enough wriggle room if deluged by speeding claims.
Image the situation if it under reported by 10%.
Somewhere on the forum ( along while ago) someone did have a fix for this as they had changed the diff and it involved resetting or selecting a value on the speedo. I cant find the post but maybe Simon can help.
