There are usually a few extra characters on the sidewall of the tyres a car left the factory wearing, and hardly anybody knows what they are for. A star, or MO, or the letter N followed by a number, or AO. Those are manufacturer approval marks, and they say that this specific tyre was developed alongside that specific car and signed off for it. It is not decoration and it is not marketing. A tyre carrying the mark can differ from the identically named tyre sold without it in casing stiffness, in rubber compound and occasionally in the tread pattern itself, because the car maker asked for changes to get the ride, the steering weight and the noise where they wanted them.
That matters when a full set is being replaced, and it matters most on a car that has always felt right. Fitting the same brand and pattern without the mark is not a mistake, and on some cars the unmarked version is the better tyre. But it is a different tyre, and if the car afterwards feels a little busier, noisier or less settled than it used to, this is very often the whole explanation. Read the entire sidewall rather than just the size before you order anything, and tell us the letters when you ring.
The tread has probably been telling you for a while. Tell us the size off the sidewall and a time, and the van comes to the car with the tyre, the fitting gear and the balancer on board, to the drive in Portman or wherever it is parked. Tight in Montagu Square, easy in Baker Street: neither stops the job. Level in Devonshire Street, sloping in Portman: we set the car straight first.
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Cars around here tend to be kept a long while and get their tyres replaced one or two at a time, by whoever happened to be convenient at the time, which is how a car ends up wearing four different specifications without anybody deciding on any of them. If yours has lived on the streets around Portman and the High Street for a few years, it is well worth walking round it with a torch and reading all four sidewalls properly before you buy anything. What you find often explains a car that has never quite felt the same since some forgotten Tuesday.
A set went on elsewhere in the right size and the right pattern but without the approval mark. The owner could not put a finger on what had changed and was quite sure something had. The marked version went back on at the next change.
A car had one approved tyre and one ordinary one of the same name on the front. Same size, same tread, two different casings, which is two different amounts of grip at the same end of the car.
A driver photographed all four sidewalls and read the letters out over the phone. It took a minute and it meant the tyres that arrived were the ones the car was actually designed around.
They are approval marks showing the tyre was homologated for a particular manufacturer. A star is one maker, MO another, N with a number another, AO another again. Each indicates the tyre was built to that car maker's specification rather than to the tyre maker's general one, even where the name and size are identical.
No. It is a choice rather than a requirement, and plenty of excellent tyres carry no mark at all. The reason to keep it is continuity: it is the surest way to get back the ride and steering the car was signed off with. The reason to leave it is that an unmarked alternative sometimes suits how you actually drive better.
Nothing dramatic. The car's systems were calibrated with the approved tyre's behaviour in mind, so what you tend to notice is character rather than function: a slightly different steering weight, a different noise, a firmer or softer edge over bumps. It is a change, not a fault, and worth making on purpose.
They are ordinary stock lines and no dealer is involved. Tell us the letters when you book and the right ones travel out with us to wherever the car is standing, whether that is a permit bay or the kerb outside. The choice is made before we set off rather than at the roadside.
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