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I have one damaged tire that needs to be replaced on my BMW 320i year 2014. Can I buy only one new and expect no annoyances from RDC? The point being the old tire has a worn a couple of millimeters - and paired with a new (same make & model) one on the same axle, the circumference will be different on the two wheels. Any thoughts or ever better; hands on experiences?

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  • Note that its never good to have two tires with a difference in size on the same axle, but that depends on just how much one of the tires is worn, and makes a bigger difference on a driven axle.
    – JPhi1618
    Commented May 11, 2016 at 14:04
  • Does the system on your car use in-tire sensors for pressure, or does it work based on the wheel speed of each wheel?
    – JPhi1618
    Commented May 11, 2016 at 15:33

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There are two types of tire pressure control systems.

  1. The simpler and cheaper one compares the average rotational speed of all four wheels, which is already measured by the anti-lock system. If pressure is low on one tire, the tire will bulge and sit a bit deeper on the road. The effective radius / circumference is smaller, and so the average rotational speed is higher compared to the others. This system advises you to check the tire pressure, but doesn't know it. Also, it doesn't notice if all tires are on the same, too low pressure.
    A completely worn tire can have 7-8mm less radius than a new tire (for normal cars), and the pressure control system will for sure trigger. However, nobody knows where the exact threshold is. A small difference may not trigger the system, but reduces the sensitivity to real pressure loss on the bigger wheel.

  2. The "better" one has a pressure sensors in each wheel, and transmits the data wireless. While this is quite expensive, it gives exact pressure values and does not rely on rotational speed. You could mount completely different tires and won't get a pressure warning.

Since March 2014, all new BMWs are equipped with the second system, so you won't get pressure warnings.

However, wheels with different tread depth on one axle can be dangerous in extreme situations, as they can have different traction. Depending on how much the old tires have worn, I would buy one or two tires.

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