Was speaking to a guy at a local tyre shop about this; he had two stacks of tyres, both identical in terms of measurements (245/35/R18) but when all 4 were stacked, one of the piles sat a few inches higher than the other...every brand is different. (Admittedly this only shows width variations, but the same applied to section height).

There's no point working out that you need a 235/45/R17 and then buying a tyre from a manufacturer that notoriously undersizes.

The moral of the story is that if you're that worried about your rolling diameter affecting your speedo, you should take a tape measure and physically measure the total rolling diameter of each tyre you are thinking of buying...or just buy a decent GPS.

/mah slightly off topic $0.02

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Ryan