I'm talking about rolling radius, not width of tyres. For the car to understand if the rear wheels are spinning it monitors the speed of the front and rear wheels (rear wheels spin faster, then TCS will kick in). Now as standard, the rear wheels have a bigger rolling radius than the fronts (IIRC there is a 3% difference). For the TCS to keep working you need to factor in this difference and keep it the same. As soon as you change it (more than a percent or two) it will get confused. Now the absolute rolling radius doesnt matter, what you need to keep in mind is the percentage difference front to rear wheels. I've written a detailed comparison before so give it a search and you'll find the numbers. You can run non OEM rolling radius so long as you keep this relation of front to rear correct