Can someone explain to me the tire "rating system"

[optimistic]

For example:

205/40HR17

Anyone know what it means?

 

[freebee]

Width/Aspect ratio, wheel diameter.

 

[optimistic]

What does the aspect ratio doos?

 

[Lvis]

Aspect Ratio: An expression representing the height of the sidewall of a tire in terms of a percentage of tire's width.

from some google link.

 

[optimistic]

Thanks! So the lower the aspect ratio, the skinnier the tire?

What kinda width to I need for wheels the width of of 7.5.

 

[prodigy]

http://www.tirerack.com/tires/tiretech/general/sidewall.htm