Q&A

 Master

I would first inspect the vehicle speed sensor, because if that is wearing out then the mph will be off. Ever get any check engine lights on, if so, then you may have a Vehicle Speeds Sensor code. Get this issue resolved and if that is all with in spec, then have a shop with a Mini factory scanner check the accuracy of the Speedo, it may be failing, because I have not really heard of a Speed losing its calibration, unless there was some glitch that told the computer system that all of a sudden you have different size tires, that is possible, but it would be unusual.