Wandell,
You seem pretty well versed in tires and rims. In a prior thread you had mentioned that wheel size will not effect size of tire you can run. Im lost on this. Lets say I wanted a "taller" tire, in other words a higher aspect ratio say 305/65 instead of 305/60. I would think that if I go from a 20 inch rim down to a 18 inch rim it gives me two more inches of height that I can apply towards a taller tire?? What am I missing??

 

Well im not familiar with metric sizing, but this might help? If you had a 35" tire with 20" wheels, and wanted to go to 18" wheels, that doesnt mean you can use 37" tires. The tire size is generally unaffected by wheel size, because the 35" in a 35/12.5 r20 tire is still going to be the same 35" height as in a 35/12.5 r18 tire. (you just get more sidewall rubber with the 18" wheel)

Hope this helps? :P

 

A 305/55/20 pretty much equals a 305/65/18
You don't gain any actual height going to the 18's, just more sidewall on the tire.

Does that make sense?
(I know, I'm not Wandell)

 

RIght. Basically, if you can fit a tire 33 inches tall and 12.5 inches wide (for example), it will not matter if you have a 17, 18, 20, 22 or 24 inch wheel. The width and height of the tire will be the same. The only difference is a 33x12.50-17 will have more tire sidewall than a 33x12.50-24.