Subject: Vehicle tyre size & the effect on ABS braking system.

Vehicles involved: All ABS Equipped Vehicles

These vehicles may exhibit unusual ABS system behaviour when the OEM tyres are replaced with ones that vary from the original size. This behaviour can show up as excessive tyre chirping or erratic ABS system modulation during a panic stop. In severe cases the ABS warning light will illuminate, set a trouble code and disable the ABS control system.

Replacement tyres should always be identical in size to the original tyres or be a factory recommended option for that vehicle. Tyre size and final drive ratio ( transaxle or differential)
determines the number of teeth the manufacturer selects for wheel speed sensor ring (toothed ring). The sensor ring is located on a number of different components such as front or rear rotor hub, rear axle flange, outer CV joint, tail shaft of the transmission or on the ring gear in the rear differential. These components rotate proportion to wheel speed. Wheel speed sensors are affixed to a stationary component near the sensor ring.

The sensors work in conjunction with sensor rings, to produce an AC voltage signal. The ABS control module utilizes this signal to interpret the rotational speed of the tyre and wheel assembly. Deviating from the factory recommended tyre size affects the rotational speed of the sensor ring which in turn causes excessive variation in the voltage signal to the ABS controller. A tyre smaller than OE will generate a signal that causes the ABS control module interpret that the vehicle is travelling at a higher rate of speed than it actually is. The opposite is true for a tyre that is larger than original equipment.
Bottom line, don’t install tyres that are of a different size and preferably replace worn tyres in matched sets of four.