Porsche 928 ST1991 Owner's Manual page 30

Note
The control unit of the ABS brake system is
set for standard tire size. If non-standard
tires are fitted, the control unit may misinter-
pret the speed of the vehicle, because of the
variant data it receives from the sensors on
the axles.
If the difference in rolling radius exceeds ap-
prox. 6 %, the control unit deactivates the
ABS system and the ABS warning lamp
lights up.
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