Originally Posted by BGTV8

The VDC system expected same OD (within ~20mm OD).

If tyre diameters are mis-matched and outside the VDC system tolerance, it pops the slip light on the dash because it "thinks" that tyres on one axle are rotating faster than the other and it wants to intervene.

There is no precise figure for the tolerance, but generally accepted to be around 20mm (0.8-inch).