For the times you need to find out what size tire was OE on a Car or Truck . For the times maybe looking to see if a rim or tire from a ?? yr car will fit . Came across this web site.
Wheel-Size.com :: Reference guide for car wheel and tire dimensions such as size, offset, PCD
At the home page choose your vehicle.
At the Rim & Tire Size Calculator put the size tire you need, or the other side of page put in another size to see if it matches your size in height. On this page upper left side you can make your answer appear in Inches or Metric.
Also change offset of rim, width , a little of everything to see how it fits on car.