KEY POINTS:
Ever wondered what the numbers and letters on your tyres mean? They relate to the rating of the tyre - what usage it's designed for - and its size.
My hypothetical tyre says P225/45R16 on its sidewall. The P denotes it's a tyre for a passenger car. The 225 measures the section width of the tyre in millimetres.
Tyre width is a balance between grip and economy. The larger the number, the wider the tyre. The wider it is, the greater the grip; the narrower it is, the less resistance it offers and the better the effect on fuel economy.
Older cars tend to have narrower tyres. Modern mainstream cars find a happy average and use the same tyres front and rear, while performance cars often use wider tyres on the back to grip better when accelerating. The Maserati GranTurismo, for example, uses 245-width on the front, but 295 on the rear.
The 45 is the tyre's aspect ratio, expressed as a percentage of the tyre's width. Put simply, it tells you the height of the tyre's sidewall from rim to tread.
The 45 means this tyre's sidewall height is 45 per cent of its width, and the smaller this number is, the wider the tyre is compared to its height. So a 245/35 tyre will have a lower sidewall height than a 245/75 tyre.
Performance tyres feature low sidewalls for stiffness, to better suit them to hard cornering. But stiffness compromises the car's ride. If comfort is important, you'll appreciate the added flex of a deep sidewall.
The next letter indicates the tyre type - ie. R for radial - and the speed it's rated at, while the 16 is the diameter of the wheel in inches.
Your 16-inch wheel could carry a low-profile (short sidewall) tyre if you're a sporting driver who's used to a firm ride, or a high-profile tyre if comfort is important to you.
If you need a replacement tyre, it's vital to ensure its size and rating, and preferably its tread, matches its opposite number.
