Is a tire diameter measured from the very outside of the tire, or the inside where the wheel fits in?

Question

I am looking for tires and wheels separately, and want to know which measurement they are talking about on tire websites.

Answer 1

The number you are looking for is the last number in the series of numbers. P225/75  15, means it fits a 15" rim.

On oversized pick up tires both the diameter of the tire and the diameter of the wheel will be in the number, but the last number is the wheel diameter size

Answer 2

Most likely, they are referring to the internal diameter, where the tire mounts to the wheel. For instance, if you are looking at a 225/45/17 tire, that tire would fit on a 17" wheel (the last part), have a width of 225 mm (first part), and have a sidewall height of 101.25 mm (.45 * 225).

So, to get the outer diameter, you would add the internal diameter to the height of the sidewalls (remember there are 2 sidewalls contributing to the height).

Answer 3

I think what you are asking is the rim size that a tire fits.

See the link below.  It is a very good article that explains how to decode what the tire size means.