Currently have a car with 16" rims, tire size of 195/50R16. Will soon be getting spare 15" rims for the car. What tire size will I need? I assume it will be 195/??R15, yes? What's the number I'm missing...?
I know next to nothing. I am frequently wrong.
You want to keep the CIRCUMFERENCE or DIAMETER of the new wheel/tire combination as close as possible to what you have. You may or may not want to keep the tire width the same, and the wheel width and offset are also important.
For any given wheel width there is a range of tire widths that will fit. For any given tire width then you have to pick the aspect ratio that will provide the tire diameter that you want. You should be able to find tire calculators online that will help you to play what-if's with tire sizes. One reason you will need to do this is that some brands don't produce tire models in all combinations of width and aspect ratio.