yep I know about that too but thanks for giving details its a worthy point

 

Technically odog is right, it's simple geometry. If you take an 8.5 inch wheel and put a 225 tire and a 235 tire the sidewall will change even though the sidewall height is the same (40). It is because you are stretching the tire (225) wider. Trust me I know, I've seen it, though it is only a 5 mm diff which you can barely tell, but it is different.

 

225 section width
40 aspect ratio... 40% of the section width gives you your sidewall hieght in mm or 225*.4= 90mm tall
17 rim size

the 225 will give 4 mm more wheel wel clearance on each sideand lower the car 4 mm (you wear off more than this from when the tire is new untill the tire is needing repalced)

a 225 has a 90 mm sidewall and a 235 has a 94 mm sidewall, you can figure this out by multiplying 225*40% (which = 90 mm) and 235*40% (which =94 mm)

going to a 225 will have no effect other than to the wallet.

The aspect ratio (the middle number) is always a percentage of the width 35 is 35%, 40 is 40%, 70 is 70%

Rim width only determines what width tire (225 or 235 or so on) can fit on the wheel safely.

By taking the aspect ratio eqation and mulitplying by two (because there is a sidewall on each side of the wheel)then convertingit to inches you can determine the overall diameter by adding this number to the rim size to figure out desired tire size when changing rims or tire sizes if overall diameter is a consern.