General rule of thumb is from what a tyre fitter said to me is. Your don't want to go 1 inch wider than the rim width.

Reading through this thread about fitting 235 on a 06 sti looks to be normal.

The car has some cheap 225 china tyres on currently and they are so noisy compare to a potenza.
It's for my brother and he wants to change them to 235's with a decent brand tyre. Not looking at anything track worthy ATM but something that's still good for the street. Also would like to know if the speedo will be out or spot on. I was thinking he should go for potenzas all around with a 235 45 17. Any feed back is appreciated. Cheers.

width of tyre won't have any effect on the speedo. That is altered by changing the rolling diameter of the tyre. (ie: going from 17" to 18" rim and also the profile of the tyre changing (the 35/40/45) number in the tyre size

technically the width will actually affect the rolling diameter because the sidewall sizing is a percentage of the width of the tyre

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2000 WRX - Grip King
1997 Sileighty - Gate Monster

All good ive got 18" x 8s with 235 x 40s on my 03 STI lowered dosent rub at all speedo reads correct looks better with the wider tread from the rear too.

Thanks for your help guys. I'll tell my bro to order 4 x Potenza S001's in 235 45 17's. Decent road tyres IMO.

If keeping the same aspect ratio (eg 40) 235 / 225 = 4.5% increase in rolling diameter, well within legal limits. Your speedo will now read closer to true speed, instead of always high as set from factory.

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A nation of sheep will beget a government of wolves.

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

they are slightly wider depending if the tyre has a rim protector sidewall

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2000 WRX - Grip King
1997 Sileighty - Gate Monster

Yeah different brands will 'appear wider' because of the differences in sidewall.

This also applies to putting tyres on the rim. Some brands will appear stretched on the rim when another brand would not.