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You can run a square setup with 18” or 19”, it doesn’t matter. 18” will be more comfortable with the cushioning of more sidewall and less prone to tire/wheel damage from potholes.
Below is the process that I followed along with the advice of Alohasurftoad
Start with the actual tire size and model that you want to run. That will lead you to the wheel width. And finally the offset ranges that will fit and possibly spacer sizes to make fine adjustments. Getting the car aligned is important too.
Square 255’s fit well. Some guys have managed 265’s but that can cut it close unless all variables are perfect.
I wanted 255 in the summer Michelin Pilot Sport 4S. 255/40-18 matches the 26.0” stock diameter of the 225/45-18 runflats. The MPS4S is more oval in shape/profile so an 8.5” wide wheel looks like a perfect fit with the sidewall. If I had chosen a more square profile tire like the All Season Continental DWS06+, then a 9” width wheel might look better.
So my tires would be MPS4S 255/40-18 and the wheels would be 18x8.5”. So what about the offset?
Wheels with custom offsets are incredibly expensive. So it’s a matter of going through many many wheel choices to see what look you like and what offsets they are made in.
Wheels have a huge price range depending on their quality, strength and weight. There are different manufacturing methods to attain these qualities.
For 18x8.5”, a common offset made is ET35. The front wheel well on my F30 can just fit an ET42 without rubbing against the suspension strut, but that would certainly look recessed into the wheel well. That’s okay because spacers can always be used to pull the wheel out a bit.
ET32 or ET30 might just fit without hitting the fender lip (also depending on whether the car is lowered and exactly what the alignment settings are.) But ET30/ET32 would leave no room for adjustment because spacers only make wheels poke out more from the wheel well.
After considering maybe 100 wheels my favorite were the APEX VS-5RS. For the 18x8.5” the only offset in my range was ET40 which was fine. These wheels look incredible, are super strong forged and are super light at only 17.2 lbs each. In fact this wheel/tire setup saves almost 10 lbs per corner, huge reduction in rotational mass that provides improved acceleration, braking, handling and gas mileage. It’s simply less work to spin, stop and control a much lighter wheel & tire.
I ended up measuring with a drop string to perfectly align the tire sidewall flush with the fender lip. It took 3mm spacers in front and 5mm spacers in the rear. Turner Big Pad spacers were by far the best because they match the pad size of the APEX wheels (and my winter BMW 400M wheels). Photos attached.
Hope this helps!
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