BMW mobile tyre fitting

A five-pointed star moulded onto the sidewall identifies a tyre developed with BMW for a particular platform. It is not a brand logo and it is not decoration: the casing stiffness, compound and often the tread pattern differ from the same tyre sold without the mark.

Mixing a star-marked tyre with a non-marked one on the same axle produces a car that behaves differently left to right, and on an xDrive model it also introduces a rolling radius mismatch the transfer case has to absorb.

BMW has fitted run-flats as standard across most of its range for years, and the majority of its cars are supplied with no spare wheel at all. If your car has run-flats and one deflates, the reinforced sidewall will carry you roughly 50 miles at up to 50mph - enough to get somewhere safe, not enough to complete a journey.

It also means the repair question is usually settled before we arrive: once a run-flat has been driven on while deflated, the sidewall support has been heat-damaged and the tyre is scrap regardless of how it looks. Replacing a run-flat with a conventional tyre changes the ride and removes the safety margin the car was designed around.

Many rear-wheel-drive BMWs run wider tyres on the rear axle than the front. That means the wheels cannot be rotated front to back, that a "set of four" is really two pairs, and that ordering by model alone is not enough - we work from the registration and the actual size on the sidewall.