[johnung]

Originally Posted by triax37

Resurfacing this to ask about 235/45/18 tires - any reasons to avoid these? The diameter starts at 26.33" (1.36% larger than 225 tires and 1.13% larger than 255 tires), but as the 10/32 tread decreases to 9/32, my calculations suggest the diameter would decrease to 26.264", or just 1.01% larger than 26" tires. Other data:

Sizes (width, sidewall height)
- 225/45/18 - 8.86", 3.99"
- 255/40/18 - 10.04", 4.02"
- 235/45/18 - 9.25", 4.16"

Yes, I'm totally over-thinking this! Hahaha!!!

Yes you are definitely overthinking. Upgraded square tire size options for stock 225/45-18 tires over stock 18x8.0 ET34 stock wheels include:

235/45-18
245/45-18
245/40-18

Can’t really go wrong with any of them.

Stock tire diameter for 225/45-18 is 26.0”, so that’s where speedometer and odometer are calibrated.

My favorite tire size of those three is 245/40-18 for the best handling. It’s diameter is about 25.7” which means that at highway speeds the car is actually going 1-2mph slower than what the speedometer indicates.

So if you pick a size with a 26.3” diameter then that will have the opposite effect of at highway speeds actually traveling at 1-2mph faster than the speedometer indicates.

245/45-18 has a slightly higher sidewall so it’s probably the most comfortable ride.

255/40-18 tires are dead on 26.0” diameter but don’t fit on an 8.0 wide wheel according to tire manufacturer specs. So you would need 18x8.5” or 18x9.0” wheels of about ET35 offset

Hope this helps!

[triax37]

Originally Posted by johnung

Hope this helps!

Awesome - thanks so much!

[closetwagonlover]

I ditched my runflat 225/45/18 for non runflat 245/45/18 and what a difference in comfort. Plus the sidewall wheel protection. Bumps feel way less harsh. Thicker rubber looks better IMO too

[keithyo88]

thanks [johnung]: your comments confirm my decision:
Forged V-5RS
front: 18x8.5 w/ 245/40r18 ET:40
rear: 18x9 w/255/40r18 ET:30

[alohasurftoad]

Originally Posted by keithyo88

thanks [johnung]: your comments confirm my decision:
Forged V-5RS
front: 18x8.5 w/ 245/40r18 ET:40
rear: 18x9 w/255/40r18 ET:30

with the setup above, a 7.5mm-10mm front spacer would help it look and also perform better. without the spacer, the setup dials in understeer and straight line stability, which is not as good on the curves. imo it'd be better to a square setup with one of those sizes

for a staggered setup, one of these combos are better...
18x9 ET30 front + 18x10 ET40 rear with
235/45/18**+265/40/18* or
245/40/18*+265/40/18* or
245/40/18*+275/35/18 or
255/40/18+265/40/18* or
255/40/18+285/35/18

*light stretch
**medium stretch
bold is what i'd get

[keithyo88]

thank you for the insights. so do these squares make sense?
front: 18x8.5 w/ 245/40r18 ET:40
rear: 18x8.5 w/ 245/40r18 ET:40
or
front: 18x9.0 w/ 255/40r18 ET:30
rear: 18x9.0 w/ 255/40r18 ET:30

front wheel has about 1-3/4" top of tire to wheelwell, rear about 1"

[alohasurftoad]

Originally Posted by keithyo88

thank you for the insights. so do these squares make sense?
front: 18x8.5 w/ 245/40r18 ET:40
rear: 18x8.5 w/ 245/40r18 ET:40
or
front: 18x9.0 w/ 255/40r18 ET:30
rear: 18x9.0 w/ 255/40r18 ET:30
front wheel has about 1-3/4" top of tire to wheelwell, rear about 1"

either of those will work, however 255/40/18 @ ET30 pokes a few mm up front and 245/40/18 @ ET40 will be fairly tucked all-around. you can also do 255 lightly pinched on 8.5 ET40 or 245 lightly stretched on 9 ET30. imo, ET30 is slightly too low and the only remedy to poke is negative camber, which may not be desirable. whereas you can always use spacers from ET40, like johnung has.

if you browse through the apex thread linked below, it has most of the setup combinations. many with 255 or 245 @ ET30 square so you can get an idea.
https://f30.bimmerpost.com/forums/sh....php?t=1393886

[keithyo88]

Grateful for the confirmation: square 18x8.5 w/ 245/40r18 ET:40
I will get a better idea of the look, possibly add spacers and change my order tomorrow. Will update when they arrive. This is an excellent crew and thanks again.