E36 18x9.5 et31

[vollosso]

18x9.5 et31
Cut fenders, no pull.
Factory camber

What's the widest tire I can fit?

255/35
255/40

265/35?

[pDubs]

Depends on how low you are.

18x9.5 et31 will most likely need a spacer in the front to not hit the coilovers, if you are running coilovers.

I think the widest you'd be able to run with that setup is a 235/35 without rubbing or adding more camber or some fender pull.

[vollosso]

rears only. Fronts are perfect at 9"

[pDubs]

Oh okay I thought you meant all around. A 245/35r18 might be a tight fit. Why not pull the fenders a bit and add a little more negative camber than stock setup? If you want wide tires you obviously want better handling. A proper aggressive street/track alignment can make a world of difference.

[vollosso]

Laying down power and camber are enemies.

[pDubs]

Not necessarily. Not if you want to turn.

[vollosso]

Not necessarily. Not if you want to turn.
...

[MatthewH]

18x9.5 et31
Cut fenders, no pull.
Factory camber

What's the widest tire I can fit?
Those aren't going to fit the rear at all without removing the inner metal fender completely. If you're doing that I don't see where a tire width restriction comes into play. Just add spacer for the inner clearance.

I would highly suggest a lot more offset in the rear. Like +45 or more. I've got 16x9.5 +38 all around with -2.9 degrees of camber in back and about -4.5 degrees in front. Even with the little 16" wheels I have room on the inside in the back.

235/35R18 is a perfectly good tire size for those narrow wheels.

FWIW: soft, tall sidewalls really help for applying power.