Historically, early tyre technology in many countries adopted British standard, with both bias tires and radial tyres with steel wires using British standard, and truck tires still follow that standard until the era of tubeless tires.
Most European countries adopt metric tires and imported cars are mostly fitted with metric tires.
12R22.5 is British standard. Specifically, cross-section width is 12 inches (30.47cm). Rim diameter is 22.5 inches (57.1cm).
Tyres with British standard only show cross-section width, tyre type, and rim size. Compared to British standard, metric standard shows more information about the tyre, such as flatness ratio and installing rim size. Cross-section height of truck tires can also be calculated by flatness ratio.
295/80R22.5 is metric standard. Specifically, cross-section width is 29.5cm and flatness ratio is 80. Installing rim diameter is 22.5 inches (57.1cm).
Flatness ratio is also called aspect ratio, you can divide the tire section height by the tire section width to calculate the aspect ratio. Current common flatness ratio is 80, 70 and 60. It's known that the minimum flatness ratio of truck tires is 45.
Take 295/80R22.5 as an example, the flatness ratio is 80, that is, the ratio of cross-section height (from tyre thread to bead) to width is 80%.
Consequently, cross-section height equals width multiplied by flat ratio. 29.5cm*80%=23.6cm, namely, cross-section height is 23.6cm.
Diameter (104.3cm) of tyre 295/80R22.5 can be calculated by cross-section height (23.6cm) + rim diameter (57.1cm) + cross-section height (23.6cm).