There are essentially three types of size marking used for motorcycle tyres:

1. Imperial
2. Imperial Low Profile
3. Millimetric

All of these systems describe the width (W), height (H) and rim diameter (dr) of a tyre size.The Tyre Aspect Ratio drawing (on the Wheels and Rims page) shows where these dimensions are measured from and the overall diameter (D).
The height of the tyre and hence its overall diameter (D) is controlled by the Aspect Ratio (Ar).
Tyre sizes for all motorcycle tyres, except those for specialised non-road legal applications, (pure racing tyres, ATV tyres, etc.) are expressed as:
Width/Aspect Ratio – W/R   Construction or Speed Symbol   Rim diameter – dr
For older Imperial sizes the Aspect Ratio is 100% and not shown.

For example:
3.50 x 18 = 3.50” wide, 3.50” tall — 18” rim diameter

For the older Imperial Low Profile series the Aspect Ratio was 90% but as the widths of the tyres were unique, it was not shown. Where the tyre width was not unique, or where the Aspect Ratio was not 90%, the actual Aspect Ratio was shown.

For example:
4.10 x 19, where 4.10 is a unique width
4.10” wide 4.10 x 90% = 3.7” tall — 19” rim diameter
4.25/85-18, Where 4.25 is not a unique width and the Aspect Ratio is not 90%
4.25” wide/4.25 x 85% = 3.6” tall — 18” rim diameter

Perhaps the most easily understood size markings are the modern millimetric type.

For example:
110/90-18
110 mm wide/110 x 90% = 99 mm tall — 18” rim diameter

Although the mixing of Metric and Imperial units can, even in this case, cause some confusion.

The effect of changing the Aspect Ratio is to change the overall diameter of the tyre:
D = W x Ar x 2 + (25.4 x dr)
Therefore:
120/100-18   D = 120 x 100% x 2 + (25.4 x 18)    = 697.2 mm
120/90-18    D = 120 x 90% x 2 + (25.4 x 18)    = 673.2 mm
120/80-18    D = 120 x 80% x 2 + (25.4 x 18)    = 649.2 mm

Each of these tyres is 120 mm wide and fitted on an 18” rim but as you can see there is a significant difference in the overall diameters.