Purchasing a new set of tyres can be a daunting prospect for a busy car owner. There is a lot to consider – that much is obvious – but until you know the questions to ask, it’s difficult to make any kind of informed decision. The guide below is a simple breakdown of those factors that should help you to identify what you’re looking at and understand what decisions you can make on each factor. The code on a tyre wall is a great place to start, so we’ve provided a sample code as an example: Tyre sidewall code: 225/55R16 91v DOT XBFU XJJX 1315 RFT No REF
The first part of the number is the simplest, and that’s the tyre width in millimetres, which in this example is 225mm.
Narrower tyres start at around 200mm while the broader end of the spectrum would be 255 or more. Broader tyres are typically used on heavy load or high-speed vehicles as they have a short braking distance, and better speed and handling. They don’t perform as well in bad weather, however as they tend to sit on top of snow and water, causing loss of traction and a higher likelihood of aquaplaning. Broader tyres are also noisier and less comfortable than narrower tyres. Alloy wheels mitigate some of the noise of broader tyres, which is commonly seen in 4×4 and Land Rovers.
The aspect ratio comes directly after the tyre width, and refers to the ratio of the tyres width to its height expressed as a percentage. Taking the first part of our example tyre sidewall 225/55R16, the aspect ratio in this case is 55%. That means the width of 225mm is 55% of the height of the tyre.
The aspect ratio determines the traction of a moving vehicle. Deviating from the recommended tyre specifications in your vehicle handbook can affect the way the contact patch (the part of the tyre touching the road at any given time) engages with the road.
Radial or Bias (Crossply) Construction
Radial tyres are marked with an R on the sidewall, but are instantly recognisable for the 90 degree angle that separates the tread from sidewall. The distinct join between the two is reinforced by a steel belt, which also stabilises the crown. These are a common site on the roads, as they’re the better option for your average car.
Bias-ply tyres function as a single working unit, which is constructed from a network of interlocking criss-cross nylon piles. This makes the sidewalls extremely tough. You will most often see these tyre types on agricultural and industrial vehicles.
The two digits following the Radial or bias indicator are the wheel rim diameter. When purchasing new tyres, you need to be sure that the wheel diameter of the tyres matches that of your wheel rims. A tyre marked with a 16, will only fit a 16 inch wheel rim.
High Performance Tyres
The ZR rating marked on the tyre wall indicates the speed constraints of the tyre. These certified speed ratings range from 3mps to over 300 mph. Each manufacturer can vary on maximum speeds, so it is best to check.
Date of Manufacture
Run Flat Tyres
Run Flat Tyres are a cost effective way of safe-guarding against punctures on long journeys of up to 50 miles, or times when you could simply do without the upheaval. Each manufacturer has its own sidewall code for run flat tyres on the tyre sidewall and can easily be looked up online.
Tyre Load Rating
The tyre load rating, or maximum carrying capacity, is indicated in kilogrammes just to the right of the diameter. These are mainly relevant for vehicles carrying extreme weight on a regular basis, but should be considered before using a family car to transport a dismantled shed.
Further information on sidewall codes pertains to whether or not the tyre is reinforced, dry and wet type, soft and hard. These are often very specialist, but for the majority of road users, the above gives a good overview of what you are looking at to ensure a safe driving experience and confidence in future tyre purchases; whether new or used – there is plenty of choice once you know what to look for.