Porsche 928 GTS 1994 Owner's Manual page 33

If the ABS system becomes inoperative,
take your vehicle to your authorized dealer
immediately.
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 misinterpret 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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