It may depend on the brand of tire and of course it definitely depends on the type of wheel.
I have 215/45-17's on my focus SVT wheels. There is about .5" of clearance on either side right now.
Oh, I have falken ziex 512's on them. They almost seem on the narrow side for the rim so I am moving up to 235/45-17's pretty soon.
20 more millimeters is about 3/4" and if you split that two ways, inside and outside of the rim then 1/2" on either side should still fit it.
The added height will help me a bit too.
Former owner of '99 CSVT - Silver #222/2760
356/334 wHP/TQ at 10psi on pump gas!
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'05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red
'06 Mazda5 Touring, 5spd,MTX, Black