Car: 9th Gen 1.4 Petrol SE + Navi
Join Date: 13th March 2017
Location: Essex, UK
Thanked 294 Times in 264 Posts
Sounds like it is regular overreading; The easiest way to get an idea of true speed is with a GPS.
If you drive at a steady speed, see what the GPS gives you and you can work out a slight offset.
An over read is normal, as it's not permitted for the speedo to under read at all, manufacturers tend to err on the safe side and under read. with option wheels etc. this can be amplified.
Unfortunately, there isn't really an option for getting the speedometer calibrated - I did have a look in the UK to see if I could get the speedometer calibrated similarly to Police speedometers, but there isn't a service accessible to the general public that I've found.