Mopar speedo calibration is the easiest in the industry, "back when." This is because of the design of the speedo housing. The gear on the mainshaft is part of the shaft, and therefore "a standard." The plug that holds the driven gear is off center, and compensates for different sized gears. READ your shop manual, download one here:
There's a chart right in the book that shows tire size versus gear ratio for the right gear. You will have to go online or contact a tire store to convert the old sizes to the new.
