In a nutshell: The main fator in this tyre size thing is the VSS, as the whole management of all the systems(engine, transmission, ABS, ACE, etc) was ''calibrated" from factory to a VSS calculated for standard dimension tyres, increasing the dimensions without a remap to suit the new VSS is mixing up the whole algorythm
ENGINE MANAGEMENT SYSTEM - Td5, DESCRIPTION AND OPERATION 18-1-39
Vehicle Speed Signal (VSS)
The VSS is an integral part of the ECM’s overall adaptive strategy. The ECM receives the signal direct from the
SLABS ECU. The SLABS ECU is not connected to the controller area network (CAN) so therefore is hard wired.
Vehicles fitted with automatic transmission have two vehicle speed input signals to the ECM. One signal is from the
SLABS ECU and the other is from the automatic transmission ECU. The ECU compares these speed signals.
The ECM also receives transfer gearbox information. This allows the ECM to take in to account the vehicle being
driven using low range gearing and compensate as necessary. The signals generated by the SLABS ECU for manual
transmission, and by the EAT ECU for automatic transmission are received by the ECM in the form of a PWM signal.
The frequency of this signal changes in accordance with road speed.
The input signal for the SLABS is measured via pin 13 of connector C0658 of the ECM. The SLABS ECU generates
a PWM signal switching between 0 and 12 volts at a frequency of 8000 pulses per mile.
For vehicles with automatic transmission the input signal for the EAT ECU is measured via pins 32 and 35 of
connector C0158 of the ECM. These pin numbers provide a bi-directional communications link using the CAN data
bus.
In the case of a VSS failure on vehicles with automatic transmissions the ECM applies default values derived from
the EAT ECU. There is no default value for manual transmission vehicle.