Hi Jason Whatever the outcome regarding your last point the speedo calibration is ultimately affected by the rolling circumference of the tyre. A speedo should be calibrated for specific wheel diameter, tyre tread width and tyre aspect ratio, therefore change to any of these specifications would impact upon speedo accuracy. I could only find US sites to offer comparisons with a quick search, but these may prove to be of interest: https://www.tacomaworld.com/tirecalc https://tiresize.com/comparison/