The previous owners of your car appear to have put the wrong size tire on is there a way to tell if the speedometer is calibrated correctly to these tires or if its calibrated to stock?

Answer 1

Unlikely there has been any change made to calibration. That said, stock speedo is likely to show faster than actual true road speed. e.g. Speedo 30mph, actual 28mph.

For a rough guide have a friend pace you with their car at agreed speeds or try streets with speed advisor displays.

As to what tyre variation is likely to do for speedo readings, have a play around with the free calculator on this site:

http:/www.tyretraders.com/Tyrecalculator