Hello Digesters;

Ben the Link-Boy here. Thought I'd pass some good 'ol life experience that only would happen to me...and my '96 M3 equipped with ASC+T. This post applies to all '96 and newer ASC+T equipped cars that are even THINKING of a wheel/tire upgrade from their stock sizes.

As most people know, the '95 M3 was equipped with 235/40R17 on all four corners. In '96, the staggered 225/45 and 245/40 was now standard. If you do the math regarding profile height, you have the following...

235/40 --> 94mm sidewall height
225/45 --> 101.25mm sidewall height
245/40 --> 98mm sidewall height

Here was the situation...I replaced my stock wheels/tires with 235/40's and forged wheels. As most know, the 235/40's don't provide much pothole protection, so when I whoomped on of my front rims, I decided to replace the worn, 235/40's on front with 225/45's for some more protection.

Guess what? AT 80mph, the ASC+T comes on, slowing me down! After 8 days with a 318ti loaner, a new DME, a new throttle body, the dealership replaced my tire combo with tires off a new '97 M3 and all was good. My applogies to Jimmy, the tech at Wide World of Cars, who got a speeding ticket on the NYS Thruway road testing my car.

Needless to say, my 225/45 and 235/40 combo translates to a 6.9% difference in sidewall height (vs. 2.9% with stock staggered setup). My rough geometric calculations say that when the car is moving at 80mph, the rear tires are rotating at 80mph while the fronts were rotating at 78.1mph. The stock setup would show 80mph and 79.2mph. I guess the DME is sensitive to this 1.9mph difference.

Anyhow, for those wheel crazy BMW owners that want to fit 295/25R20's on your M3 with 195/35R15 fronts, you're in for a surprise, too. Who knows when your ASC+T will kick on...

Lesson learned? Make sure all your tires are the same size or within the stock parameters. I'm sure even the knowledgeable Tire Rack would be clueless to this BMW electronic phenomenon.

Special thanks to Wide World of Cars for their time and effort to find my innocent bug. After hours and hours of diagnosis, they didn't charge me a single penny! They even washed my car when I picked it up! They lost their shirt on my service call, but customer service goes a long way. No wonder they're kickin' butt in the service department! I can only recommend them wholeheartedly. Now if they'd get some cooler loaners, I'd bring my car in more often!

Ben
'96 M3 <-- 80mph and happy

http://members.aol.com/benliaw <-- 'Dem dang links...