^^Actually the speedo in my e39 is more accurate than in my 135i (e82).
By German law the speedo have to report a faster speed then TRUE vehicle speed. Normally they give us a 3% to 5% fudge factor. Meaning when you are driving 100 kph you true speed might only be 95 kph(to 97 kph).
You can have your kombi(speedo cluster) coded to rear TRUE vehicle speed IF you like. I had the M3 digital speedo coded to my cluster... and for that you can either have it read TURE vehicle speed or the 3-5% buffered value. I chose to have it read true vehicle speed.
OP - are you running OE tire sizes(brands) and wheels? Larger tires can also change your speedo calibration.