C0078: Tyre Diameter
What to do
Check that all four tyres are the same size and roughly the same tread depth, and that the sizes match what the door pillar specifies. If you are running a space-saver spare, that is very likely the cause — the code should clear once the proper wheel is back on. If you have deliberately fitted a different wheel size, the car needs recoding to suit. On four-wheel-drive vehicles do not leave mismatched tyres in place: it can damage the drivetrain.
What this code means
The tyre diameter is outside the expected range.
Every calculation the braking system makes assumes it knows how far the car travels per wheel revolution. This code says that assumption has broken.
Wheel speed sensors count teeth, not distance. Converting revolutions into speed needs the rolling circumference — and if the real tyre is a different size from the expected one, every speed the module derives is wrong by a fixed proportion. Speedometer, odometer, ABS thresholds and stability control's model of the car all inherit the error.
Comparing wheels is what detects it, so the commonest triggers are anything that makes one corner differ: a space-saver spare fitted after a puncture, a single replacement tyre of a different size, or badly mismatched tread depths.
Fitting a different wheel size deliberately is the other cause, and it needs the vehicle recoding rather than tolerating. A change in overall diameter of a few per cent is enough, and the car will not work it out for itself.
On four-wheel drive it is more than an inconvenience. Tyres of different rolling radius on the same axle, or between axles, force the differentials and any coupling to absorb the difference continuously — which is a known way to destroy a transfer case.
Likely causes
Start here
Less common
- Underinflated tyre · easy
- Damaged reluctor or tone ring · specialist
- Faulty wheel speed sensor · specialist
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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