90/90 18 inch tyre instead of 90/100 18 inch.

[Krasi]

The figure after the slash show tire height relative to the width, which is the first number (in mm).

In other words 90/90 is a tire that's 90mm wide and 81mm high (90% of 90mm = 81mm), while 90/100 is again 90mm wide, but also 90mm high. And they both have inner radius of 18 inches (yeah, it's really nice how they mix up Metric and Imperial units!). Rim width does not play in this case.