So I've been doing searches and reading all the threads about winter tire and I'm a bit confused about sizes. Most people seem to be going with 17x7" rims for winter but there are variations in the tire sizes people are choosing. I've seen 205/55, 225/45, etc.

Is there one size that is best? What's the reason for people going with these different tire sizes? I don't quite understand how guys are putting tires of different widths on 7" wide rims?

if anyone can help me out as soon as possible I'd appreciate it, I'd like to get my winter tire set-up ready before my car gets here.

I'm not a tire expert so others will surely chime in...

I found this tool invaluable:

http://www.1010tires.com/tiresizecalculator.asp

If you pop in your OEM size (225/45/r18) then you can compare it to other tire sizes visually, and statistically.

I think 215/55/r17 is normally recommended because

a) it is narrower (better bite), and
b) it is cheaper size

It is 1.3% larger in diameter than the OEM rim.

205/55? I'm not sure people are buying that size?

Anyway, I went with a 225/50/r17 because

a) it is only 0.47% larger than stock, and
b) maintains the 225mm section width. I will be driving mostly heavily trafficked roads so I don't need/want a narrower tire.

You can put different width tires on one sized rim. For example I believe you can put a 225/45/r18 on a rim from 7-8.5", and a 225/50/r17 on a rim from 6-8". That info is also in the tire calculator.