Get to know your tyres better

Being a newcomer to overlanding can be daunting at times, especially if you are not a petrolhead. There are, however, some basics that everyone should know and understand, and the specification of your tyres is a good place to start.

So, why are your tyres so important? They are the only part of your vehicle that’s in contact with the terrain you’re traversing. They affect many aspects of your vehicle, from handling to traction, load capacity, fuel consumption, ride quality and even actual ride height.

How do you find out more about your tyres? It’s all mostly printed on the sidewall in big letters. That combination of numbers and letters will help you identify the tyre’s width, aspect ratio, construction, the diameter of the wheel it will fit onto, tyre load index and finally, speed rating. The information given on the tyre follows a universal pattern. For example, the combination LT 265/70 R17 121 Q provides a lot of useful information.

Service Type “LT”

Service type or tyre designation refers to the type of vehicle the tyre is best suited for. The typical lettering you’ll find is P, LT, T, C. In the case of the example “LT” specifies that the tyre is for use on a “light truck”. LT works best for heavy overlanding use and higher loads. Most passenger cars and SUVs will have “P” specification, for Passenger.

Tyre Width “265”

The tyre width is the width of the tyre, measured in millimetres. This is the part of the tyre in contact with the road. In this case the tyre has a total width of 265mm.

Aspect ratio “70”:

A higher aspect ratio means a taller tyre, suited to offroading. A lower number means a sportier tyre, suited to fast cornering. The ratio is expressed as a percentage, derived by dividing the section height of the tyre (from rim to tread) by the tyre width, and multiplying by 100.  So, our example tyre reads: 265/70. This means the tyre width is 265 mm, and the aspect ratio is 70. This means that 70 per cent of the tyre width is the actual distance between the tread and the rim or the height of the sidewall of the tyre, which in this case is 185 mm.

Tyre construction “R”:

Most tyres these days have radial-ply construction and will display a “R” here. You have other construction types, mainly Cross-Ply or Bias ply tyres, which are reserved mainly for heavy duty commercial use where speed isn’t necessary. Radial tyres are more common on passenger vehicles and generally have higher speed ratings and flexible sidewalls which provide better fuel consumption as well as an overall smoother and quieter ride. However, for overland use you might want to opt for a tyre with heavier construction such as an “LT” tyre which is robust and likely to have thicker sidewalls.

Wheel diameter “R17”:

This is the physical size of your rim measured in inches diagonally. This is an important specification for both your vehicle and your tyre. Your vehicle is a limiting factor with regards to what rim size you can get and is normally specified with your vehicle’s owner manual. The rim size does affect the overall tyre size and even ride height. While these are usually specified by the manufacturer, and there is little room to go bigger or smaller, as a general rule taller rims sizes of 19 inches and above are less good for overlanding, as this almost guarantees shorter sidewalls. This is better suited to faster vehicles and ensures cornering stability and less tyre deflection. A better compromise between on-road handling and off-road comfort is to opt for 15-18-inch rim sizes.

Load index “121”:

This is an index number referring to the maximum load weight each tyre is capable of carrying. The index ranges from 62 to 126, with 62 having the weaker load index and thus only able to carry 265 kg on each tyre compared to the tyre with a load index of 126 which can carry 1700 kg over each tyre. The average load index for most off-road tyres is between 116 and 121. This means you have a range of about 1250 kgs to 1450 kgs depending on the tyre. You can also find load index tables on the tyre manufacturer’s website.

Speed rating “Q”:

Your speed rating is given as a letter and determines your maximum tyre speed when its correctly inflated and being used within the load. The most common speed ratings for off-road tyres are Q, R, and S. Q gives you a maximum top speed of 160 km/h, R a max speed of 170km/h and S a max of 180 km/h. The rating is affected by multiple factors. A higher speed rating can often translate into better grip and less road noise. Like the load index you can find a table for speed rating on the manufacture’s website.

More crucial information

Those big numbers aren’t the only valuable information you can find on a sidewall. Also, in the smaller print you can find the tyre’s date of manufacture, ply ratings, construction, and so on. How to choose? Your vehicle type and typical pattern of use should guide your tyre selection.

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