We appreciate your interest OP, and you raise a great question in regards to the suggested tire sizes for the staggered wheel configuration at hand.
As a benchmark, here are the standard wheel and tire specifications everyone is likely familiar with depending on how the car was optioned from the factory.
Style 218/219
F: 18x8.5" ET29 - 245/40-18 (25.72" tall)
R: 18x9.5" ET23 - 265/40-18 (26.35" tall)
^ Difference in height from front to rear = 0.63" / 2.39%
Style 220
F: 19x8.5" ET29 - 245/35-19 (25.75" tall)
R: 19x9.5" ET23 - 265/35-19 (26.3" tall)
^ Difference in height from front to rear = 0.55" / 2.1%
The term OEM+ may take on a slightly different meaning from enthusiast to enthusiast, but we consider it to be an incremental yet meaningful improvement over stock, without drastically deviating from the original intended design/functionality. The two staggered APEX fitments that we consider to be OEM+ encompass wheels that are 1/2" wider than stock, front and rear. You will notice the width and offset specifications are almost exactly the same as the ZCP/GTS wheels which I think is a testament to the OEM+ label.
There are many benefits going with an 18" setup, so we are glad to hear you are thinking of going this route. With that said, the tire options for our 18" OEM+ configuration are slightly more complicated in comparison to our 19" setup. You are forced to make a compromise on either slightly short tires in the correct widths, or slightly stretched tires in the correct heights. Both tire configurations will work, but neither are ideal if we look at them with a critical eye.
APEX
F: 19x9" ET28/30 - 255/35-19 (26.03" tall)
R: 19x10" ET25 - 275/35-19 (26.58" tall")
^ Difference in height from front to rear = 0.55" / 2.08%
APEX
F: 18x9" ET30/31
R: 18x10" ET25
Tire Option 1 - The perfect widths, but a little short (more specifically up front). You can still get away with running these no problem, as both the front and rear heights have decreased resulting in a similar front to rear ratio in comparison to stock.
255/35-18 - 25.03" tall
275/35-18 - 25.58" tall
^ Difference in height from front to rear = 0.55" / 2.16%
Tire Option 2 - Slightly slim, but the perfect height as these are the OEM tire sizes after all. If the tire model you plan to use runs on the wider side, these tire specs actually end up working out quite nicely.
245/40-18 - 25.72" tall
265/40-18 - 26.35" tall
^ Difference in height from front to rear = 0.63" / 2.39%
When it comes to a staggered 18" setup for the E9X M3, we are most in favor of the "Performance Street & Track" configuration which encompass wheels that are 1" wider than stock, front and rear, and are still considered to be an easy bolt-on fitment with no modifications required. By running these wheel widths, you can properly support 265/35-18 front and 285/35-18 rear tires, which are proven to work very well on this chassis.
APEX
F: 18x9.5" ET22 - 265/35-18 (25.3" tall)
R: 18x10.5" ET22/27 - 285/35-18 (25.85" tall)
^ Difference in height from front to rear = 0.55" / 2.13%
I hope you find this helpful, and please let me know if you have any follow up questions. Our 18" ARC-8 / EC-7 Group Buy is only live for a few more days
- Ryan