So going from 40/18 to 45/18...

That will stand me approx 1 inch taller? I could use that to help front lip scraping. I'm slightly lower on R springs (don;t fancy going back to stock springs to raise the height or to remove my front lip) and have 7mm front spacers and 15mm on the rear. Any concerns with tires rubbing the wheel well? Apart from riding taller, any major handling changes for road use (doubt it but thought I'd check), fast cornering and for spirited occasion B-road drives. No tracking. Thanks all.

It wont be 1 inch taller. Its a circle so the increase is split by the opposing sides of the radius. The diameter of a 235/45/18 vs a 235/40/18 is about .9 inches or 23mm, so the ride height on that example would increase by half of that so .45 inches/11.5mm.

Thanks for the clarification. Always learning. Any thoughts in terms of wheel well rubbing and differences to driving/handling characteristics?

Its a small difference, so I cant imagine it being any issue at all. You should be able to see if you have that sort of clearance visually by moving the wheel around and getting under the car to see what the current clearances are now. Your current setup would have to be completely maxed out with essentially no available room left in order for 1cm to start hitting things

Thanks again for the share of knowledge. Much appreciated.

The calculation that results in the 0.9 inch difference assume, as in ICNU's post, that the tread width (235 mm) is the same. The 40 and 45 numbers are the sidewall height as a percentage of the tires tread width, so increasing the tread width would increase the sidewall height if the ratio remained the same. With 235 tread width, moving to 45 from 40 increases the tire circumference by about 2.9 inches, or 3.6 %. Your speedometer will read low by the same percentage. I am not sure where the Porsche nannies start to get confused, but you might want to research.

Yes, my assumption was that the sidewall profile change would be for both front/rear which would keep things at practically the same rolling bias between them as the factory setup. Its a pretty high tolerance the best I can tell, at least on my 981 . I have a 3.5 inch circumference stagger on my current setup vs stock (stock is 2.6). There has been no issue with ABS/traction control false intervention so far. My stock setup had a 3% circumference difference between the front and rear, and I'm currently at 4.1% Here is the site that I use: Tire Size Calculator

LOL, I was using the same site.

on my 2010 BS, I run stock 18" wheels summer and winter, summer tires are 40's and winters are 45's (stock widths).The winter wheels are a set of stock 2014 Cayman base wheels and tires, and only difference in specs is the 45 ratio sidewall.I got the winters with about 3,000 miles on them.The winters fit fine in back, and fit in front but just barely. They are about the thickness of a businss card away from rubbing on the shock towers (inside) but look fine on outside. No rubbing but I'm not sure about fitting abrand new tire on there. The snows are supposed to be about exacty the same dimensions as summers, but may have a different shape at the outer edge (thicker) than the summer profile. If new tires just rub slightly, they could be shaved a few thousandth's by a tire shop. I can't comment on handling differences, because of the snows, but the combination of snows and taller sidewalls definitely gives a softer ride on rougher winter roads so I would expect a slight reduction in responsiveness.

Thanks much for this feedback. I'm on spacers and lowered shocks.R springs. Not sure if that will compound the potential rubbing concern.

I see you are using wheel spacers f & r, 7mm and 15mm. If you are rubbing on the front spring, just get a 10 mm front spacer to push the tire out slightly. If that works great. The other option is to add the max camber to the front to pull in the outside tire shoulder so it doesn not hit the top of the fender lip. Camber plates or GT3 LCA's to do this work. I run -2.9 f and-2.5 in the rear.