1- Total Tire Diameter

The front and rear tire diameters should be the same (or very close to it). This means you'd need to run a lower profile tire on a wider wheel.

Some people say it's ok up to a 4% variance (/difference in wheel diameter); but if you run it close to that then the 4WD system will either engage more quickly or more slowly under "normal" circumstances depending on which set of tires are larger. When it does engage it'll cause a strange torque transfer because Haldex is trying to make front and rear axles turn at the same speed, so you'd get it putting more torque to the front or the rear, again, depending which end of the car has the smaller tires.

One should strive to make sure outside diameters of the tires are as close as possible and not purposely eat into your 4% margin.

2- Rolling Circumference

Rolling circumference is the distance a tire travels over one full rotation. As the car is loaded this circumference can change due to the different air pressure, sidewall stiffness etc (the ‘squish’).

Static (/still) diameter or circumference can be slightly inaccurate due to this squish. The tire will always be loaded as the car is rolling down the road and the Haldex is reading the wheel speed sensors.

Basically, keep your tyre pressure in check.

Big Thank you to VW Vortex for much of this info.