Im very familiar with the Avon RoadRider Series tires. They are a good all round tire and at a fair price usually.

It seems that a recommended (Barringtons also) and frequently used front tire on most 55-69 twins is an Avon AM26 RoadRider 100/90-18. Standard Rims on those models are 3.

Avons specs say the max rim width for a 100/90-18 is 2.75.

Why are not 110/90 x 18 Avons that are specd for 3 rims being used instead of the 100/90s ?
Are tires specd for the inside of the rims where the actual tire bead mounts ?

Also, what is the height aspect ratio of a tire when its size is only listed, as say, 3.0, 3.5 or 4.0 x (? ?).