Originally Posted by growling_boxer
like I said, going with a narrower tire will buy you some clearance, but just going from an 18 to a 17 on it's own has nothing to do with that, unless you are getting a narrower wheel. you can can also get more clearance going to a shorter overall diameter tire. but staying with an equal tire and going from an 18 to a 17 (e.g. 245/40R18 to 245/45R17), will not buy you clearance.
Yes if u r comparing same width (245/45/17 vs 245/40/18) but if u compare 225/45/17 vs 245/40/18 the 225/45/17 will have a smaller diameter due to the shorter width u need to remember the wider the tire the taller the tire will be with the same series but varies from brand of tire.
My dedicated winter tires r blizzak lm60 225/45/17 vs my 245/40/18. Since I have the performance winter tires and even though they're a 45 series r a smaller diameter by about 1" giving more wheel well clearance.