Every so often a wheel comes off and the tyre on it is not the size the car should be wearing. Usually nobody set out to do that. A size was not in stock, something close was offered, and close is not the same. The numbers moulded into a sidewall are the width in millimetres, then the sidewall height as a percentage of that width, then the rim diameter. Change the middle number and you change the overall diameter of the wheel, which changes how far the car travels per revolution.
That has consequences you can measure. A smaller rolling circumference means more revolutions per mile, so the speedometer reads faster than you are actually going and the odometer collects distance nobody covered. A larger one does the reverse and reads slow, which is the version that gathers speeding tickets. Stability systems take their wheel speed readings from the same sensors, so a mismatched pair across an axle has the car being told one corner is turning faster than the other on a dead straight road. It is the sort of fault that produces vague symptoms for years and never quite gets diagnosed.
An unusual size is worth mentioning early rather than late. Once it is settled, it is fitted and balanced where the car is parked in NW7, with the valve checked and the pressures set to the manufacturer figure. The The Hale phone and the Arrandene phone ring the same fitter, any hour. Midnight in Arrandene, midday in The Ridgeway, the same call either way.
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Cars around here spend real time at speed on the A41 Watford Way and the A1 Barnet Way, and that is the environment where a tyre outside the car's specification stops being a technicality. We read the plate, check what is on the other three corners, and bring the correct size out to the driveway or the station car park. An unusual size comes the same day rather than becoming a problem you have to solve yourself.
A car we were called out to for a rear tyre had a fifty five profile on one side and a sixty five on the other, both the correct width and both looking perfectly ordinary to anyone not reading them. The mismatch came to nearly a centimetre of rolling radius. Both went, and the pair matched afterwards.
The owner had noticed the car seemed to be doing thirty five while traffic alongside did thirty, and had blamed the instrument. It was the tyre. An undersized fitment on the driven axle had been quietly inflating every reading since it went on, and the fuel figures had been flattering too.
Size is not only about diameter. A heavier SUV needs a tyre rated to carry its share of the weight, and this one had been given a lower load index than the plate calls for. It held air and looked entirely correct. Loaded up on a fast road it was working outside what it was built for.
There is a plate or sticker on the door shut or inside the fuel flap listing the sizes and pressures the maker intends. Read that rather than reading the tyre currently fitted, because the tyre currently fitted is the thing under question in the first place.
Enough that we would not fit one. A few percent of rolling circumference is a few percent of error in everything the car measures from wheel speed, and it is a difference in ride height and gearing that comes through the steering more than most people expect.
Across an axle we keep the pair the same wherever possible, because tread pattern and compound affect wet grip far more than the badge does. Different makes front to rear is common and generally fine. Same axle, same tyre is the sensible rule to hold to.
It gets sourced and brought out the same day rather than something near enough going on. That is the entire reason for reading the plate before setting off, so what arrives is the tyre the car was designed to have on it.
It certainly can, and a mismatched pair across an axle attracts attention. The bigger issue is the years before that test, when the car has been quietly measuring itself wrong and the driver has been trusting numbers that were never true.
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