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a: tyre size

This is the width in mm of the tyre from sidewall to sidewall when it's unstressed and you're looking at it head on.

b: profile

This is the ratio of the height of the tyre sidewall, expressed as a percentage of the width.

c: rim size

This is the diameter in inches of the rim of the wheel that the tyre will fit on.

d: load index / speed rating

The majority of tyres carry coded markings on them, which correspond to their load carrying and maximum speed capabilities.

The load index is a numerical code associated with the maximum load a tyre can carry at the speed indicated by its speed symbol under service conditions specified by the tyre manufacturer.

For example, the diagram above shows 91W:

91 = Load Index (91 equates to a weight of approximately 615 kg)
W = Speed symbol (W equates to a speed of approximately 168 mph).