If you can access the calibration menus I would start by adding 10% to the calibration readout, in fact I would add more so that it reads over and gives you a safety margin. If it's not right you can subsequently make any alterations by comparing with the GPS reading but as you have accessed the readout it looks like you have the capacity to make the neccasary changes.
Can't be any more helpful unfortunately as whilst I have the same issue with my VDO speedo I can't access the menus to make any changes, yet.
