Going wider (Plus Zero) will not change the speedo reading. You are correct that to change the speedo reading you need to change the diameter of the tire or more importantly its revolutions per mile. A larger tire will cause the speedo to read lower. So the speedo in the GenIII Prius reads approx. 1mph to 1.5mph too fast from the factory. Installing a smaller diameter tire like the 215/45/17 (24.7" diameter) will cause the speedo to read approx. 2mph too fast. Installing a larger diameter tire will cause the speedo to read slower. Fr some reason, the 205/50/17 (25.1" diameter) actually corrects the speedo problem yet it is only 0.1" larger than the OE 195/65/15 (25" diameter). I think it is due to the more robust construction of the XL rated tire. I could be wrong, however. In a nutshell, you are correct.
