I just thought that I would pass on a trick that I have been using for many years....
To find the true rolling diameter/circumference just place the tire on a flat level surface while installed on the vehicle at the correct operating weight and air pressure. Take a piece of chalk or anything else that will leave a mark on the flat surface. Roll the vehicle forward or backward at least one tire revolution and measure the distance between the two marks on the flat surface.
This will give you the exact rolling circumference of your tire,
Check the front and the rear tires the same way and get out your calculator to check the ratio....
Hopes this makes the process easier,
Bob