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I was looking at a pair of 17" rims but the markings on the tire state 215/45R18. This threw me off because I thought the last number (18) represents how big of a rim the tires can fit on.

Is there something I'm missing in regards to tire and rim size?

I've heard of people "stretching" their tires before but I don't know much about it. Also, these are rims on a new car at the dealership (2018 Subaru BRZ tS). I was under the impression that the STI rims that come with this car were 18's, but now everywhere I look online they seem to be 17's.

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  • Have you checked the rims? They will have the size marked on them as well. I think you may have 18" rims while someone told you they were 17"...
    – Solar Mike
    Apr 8, 2019 at 13:37
  • @SolarMike I was thinking that, and I'll confirm when I'm back at the dealership later tonight but everywhere I look online this (fairly) specific set of rims seems to be only offered in 17"
    – Paul Warnick
    Apr 8, 2019 at 13:39

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The simple answer is yes, they do - the tyres physically won't fit otherwise.

The tyre markings are as follows: (width in mm)/(aspect ratio)R(rim diameter in inches) (load index)(speed index) - so in your case, 215mm wide, 97mm tall, 18" inside radius - it'd be impossible to fit them to 17" rims as there would be a half-inch gap all the way round!

Stretching is done by some people to fit narrower tyres to wider rims - it's dangerous as the bead of the tyre can't seal properly against the rim, and illegal in some countries (for example, it's against the construction and use regulations in the UK to fit tyres outside of the manufacturer's specifications).

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  • That makes sense and is what prompted me to ask the question in the first place. Thanks for the info on stretching too!
    – Paul Warnick
    Apr 8, 2019 at 14:16
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    Supposedly the stretching is done to 'look cool'. Personally I think it looks ridiculous, but each to their own!
    – Nick C ♦
    Apr 8, 2019 at 14:17
  • The width of the rim must also be compatible with the tire size.
    – blacksmith37
    Apr 8, 2019 at 14:40

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