Originally Posted by fatboyoz

Rob,
Original equipment tyre size for staggered 19" wheels should be: front 255 50R 19, rear 285 45R 19.
The tyre pressure warning system is sensed through the same sensors used fo anti-lock braking (ALB). If the computer sees a difference in wheel speed, it will activate the tyre pressure warning. This difference in wheel speed could be caused by an under inflated tyre (low pressure causes reduced circumference and wheel turns faster in relation to others), it can also be caused by aquaplaning. The wheels will turn at different speeds when the car aquaplanes and the low tyre pressure warning will sense this difference and set the warning light.
The last set of tyres I had (Bridgestone Turanzas) were not too good at clearing water from under the tyres, and I experienced some aquaplaning. Can't remember if it put the TPW light on though.
Another side effect of aquaplaning is the effect it can have on the cruise control. CC also uses the ALB sensors for speed control. If you aquaplane with the CC engaged, the computer will sense the wheel speed reducing and pour in more diesel to regain the desired speed. This could be a little disconcerting seeing the speed decreasing and the rev's increasing.
If your problem only occurs in the wet, I would suspect aquaplaning and not a real fault.
Colin.