Question on Tire wall size Options

[k-weaver]

Sorry for posting yet another post on tire sizes. I tried to search but came up dry (maybe I just missed it?). Anyway, I was wondering what the method is to picking the size of the tire wall when getting after market rims.

For example, I thought that you took away 5 from the tire for every inch you went up in rim size? Like stock I had 205/55/15 and after market I have 225/45/18

So my question is, if my theory is correct shouldn't I be running 225/40/18? Because I went up 3" I go down 15 in wall size? Also, I found a website that someone else posted up here. Looks like all you do is put the stock size in and then match up what the after market size would be. Here is the website http://www.miata.net/garage/tirecalc.html When I put my stock size and then played with it to get the aftermarket size to match, it said that I should be running 225/35/18.

Also, my rims are 7.5" wide and again i have 225s. I noticed that it looks like I could go a bit bigger because my rim sticks out a little. Anyone have this problem? I was thinking about getting either 235 or possibly 245 my next tire change.