Do tire and rim sizes have to match?

Question

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.

Nick C · Accepted Answer

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).

Do tire and rim sizes have to match?

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