Our current Tire 1 code is
The first number is the tire width in millimeters.
The 2nd number is the sidewall height, by % of the tire width.
So the sidewall height of a

The 3rd number (preceded by an R) is Rim Diameter in inches (yes, Inches and Millimeters used together!) This is the diameter of the rims bead seat base, not the diameter of the outer rim edge.
Inches and Millimeters used together?
Like saying you're 5'11 and 15 mill tall - Great idea!

The rim edge, or lip, extends past the tire bead seat to hold the bead and stop the tire slipping off the rim.
When we say a rim has a inch diameter, we mean that the diameter of the bead seat base measures inches.
The outer rim edge diameter will actually measure more, depending on the design.

Tire Width in Millimeters 265

/

Sidewall Height % of Width 60

R

Rim Diameter in Inches 18