sourcing tires insidie China

[chinabiker]

Hi there, I am just brwosing Taobao to find some knobby tires for my JH-150 GY-3 and I am confused.
My current front tire is 90/90-19 and the rear 110/90-17. So I know that the last figure refers to the size of the wheel in inches, the first figure is the tire width. The figure in the middle is something like the section proportion. But what does it actually mean and why can't I find any knobby tires for my bike on Taobao? Is it ok if the first and last numbers are the same but the middle one is slightly different?

Any explanations highly appreciated!

Heard of Google yet?
http://www.bridgestone.com/products/...ion/index.html

http://www.conti-online.com/generato...andbuch_uv.pdf