Changing tires to a new size or changing rear-end gears can have an impact on speedometer accuracy. This is because your speedometer is controlled by two small gears that are optimized for a particular combination of tire size, rear-end gear ratio, and transmission type. The drive gear is attached to the transmission output shaft inside the tail shaft. The driven gear is attached to the end of the speedometer cable that is inserted into the side of your transmission.
I'll admit up front that I'm a huge fan of not reinventing the wheel, so instead of trying to develop my own Mustang VIN and data plate decoder I prefer to point you to the best that I've found on the web. Here's one for Mustangs: