CAR House

Since I am going up to 8.5in wide wheels F/R, I was wondering if I could get away with doing a 235/40 18 Front and a 255/40 in the rear. Would the sidewall mismatch be a problem for the ABS or Traction control systems in a 97 840? What about the speedometer?

Thanks!

In aside, I realize I am inducing notable understeer.

that represents a 2.5% difference in circumference. The ASC system really doesn't like anything over 2%.
Why don't you go with a staggered wheel set-up, since you are already trying to run different front and rears? 245/40-18 in the front and 275/35-18 in the rear is the usual set-up, this is a 0.5 % difference in circumference which the ASC can handle, plus it looks alot cooler...

that represents a 2.5% difference in circumference. The ASC system really doesn't like anything over 2%.
Why don't you go with a staggered wheel set-up, since you are already trying to run different front and rears? 245/40-18 in the front and 275/35-18 in the rear is the usual set-up, this is a 0.5 % difference in circumference which the ASC can handle, plus it looks alot cooler...
I would normally agree, but the price on the wheels was too good to pass up. Thanks for the info!

if you are having or going to have problems, was the price really that good???

Do 255/35 x18 on the rear. Then they match:)

that is true, and it will also make you go slower than the speedo is showing. you want the "larger" of the two different sizes on the back. Although the 255 will work, the 275 is closer to the actual speed.

I recommend 18"tires in 245/40 8" Front and 275/35 9.5" Rear. No problems whatsoever--perfect match for E31's curves.

Don't whatever you do put 275/35s on your 8.5 rear rims. They are too wide and potentially dangerous. Not sure if you've already bought the 8.5s all round but look at the pain mine caused me http://forums.bimmerforums.com/forum/showthread.php?t=883737. Go for staggered 8 front and 9.5 rear and the tire size the guys are recommending, otherwise you will probably regret it.