I have 14 Ford Fusion SE, 2.0 w/ 235/45R18. I want to downsize for winter to 17". I understand the rule of thumb is to stay as close to the OEM diameter as to minimize difference with speedometer, odometer etc. Doing the math it shows that a 215/55R17 would be nearly identical in diameter. However each place I check online of store I go to they initially start with either 225/50R17 or 225/55R17. Both of these sizes stay within 2% of the OEM but still not as close as the 215. I ask them about the 215 and they do say it will work but it's not there first recommendation. Am I missing something here?
As you're changing rims from 18" down to 17" i would say its more important to get tyres which are the correct width for your new rims before worrying about the profile/OD. (You haven't mentioned it but are the rims the same width?).
At a guess the tyre shops might also be recommending the 225's because they may be a more common size.
Did you ask them why they recommended 225's instead of 215's? Surely they must be able to explain their own recommendation.
I'll say one thing to you, don't do the math ... let some place like tirerack.com do it for you. It has been my experience that one tire manufacturer's numbers (ie: 225/50R17) do not match another. There is only one true way to evaluate tire diameter and that is with the industry standard of Rotations Per Mile (RPM). When you look at tire brands/models, you'll find that each tire with the same size profile will have different RPMs. Some of those differences will be small, others will be large.
Thanks for all the input on this. Most of what I read out there talked about importance of maintaining the diameter, so that is why I was on mission to find the closest. My OEM setup, 18 x 8 (offset 55), 235/45R18 Good Year Eagle F1 Asymmetric 2. According to Good Year website, diameter is 26.3 with 793 revs per mile, load index 98, load range XL, max load 1653.
The setup that I'm looking at is 17 X 7.5 (offset 50), 215/55R17 Michelin X-Ice Xi3. According to Michelin website, diameter 26.3 with 791 revs per mile, load index, range & load all same as OEM.
As far as asking about their recommendations, Tire Rack didn't elaborate as to their recommendation, nor did Discount Tire other than both of them said it should be ok. As far as options both places offered many tire options in both 215 and 225, so both appear to be a common size.