I know what you are saying, but as written it is incorrect. The 245 is the width of the tyre and has no impact on rolling radius and hence speedo readings. The key figure is the profile of the tyre. If the profile of the tyre is 45, that means that the sidewall is 45% of the width from which and knowing the size of the wheel (17", 18") you can calculate the rolling radius
You are absolutely correct and have explained it much better than my attempt