Do you speak tyre?

Those numbers and letters on the sidewall of your 4×4 tyre aren’t alphabet soup. 

To the untrained eye a tyre has a lot of random characters peppered all over it. Knowing what they mean will empower you to make better, safer choices when replacing or upgrading your 4×4’s tyres.

THE DIFFERENT TYPES OF 4X4 TYRES

4×4 tyres can be divided in four categories, depending on the intended ratio of on- versus off-road usage. The two-letter designation usually follows the brand or range name – Bridgestone/Dueler in this case

• HP: High-performance road tyres (95 per cent on-road)
• HT: Highway-terrain (80/20)
• AT: All-terrain 4×4 (50/50)
• MT: Mud-terrain 4×4 (90/10)

TYRE INDEX MARKINGS

The text sequence above indicates size, build type and load and speed tolerance.

This tyre doesn’t have it but, some manufacturers’ size designations are preceded by a letter indicating the vehicle type the tyre is designed for. P for ‘passenger’ and LT for ‘light truck’ are most common.

SIZING UP

We’ll use the 265/65R17 112S as an example:

* 265 is the tread width in millimeters. A wider tyre has a negative impact on fuel efficiency, but improves traction. * 65 is the tyre profile and, expressed as a percentage, represents the relationship between sidewall height and tread width. In this example, sidewall height is 65 per cent of the 265-mm width. TIP TO REMEMBER: Taller sidewalls improve a vehicle’s undercarriage clearance. But if you’re increasing width and/or profile, check the new tyres don’t rub on bodywork and suspension bits during turning and axle articulation. * R tells us it’s a radial tyre, which contains multiple steel cords and provides more strength, stability and puncture resistance than older technology bias-ply designs. * 17 indicates the rim diameter in inches. * 112S, usually the last number and letter combination in the sequence, represents the load index (112) and speed rating (S).

LOAD INDEX NUMBER

Load index number (112) indicates the weight a tyre is approved to carry at maximum inflation.

You’ll need a full chart to determine the weight capacity of all tyres, but the table below should cover the tyres most overlanders would use:

SPEED RATING

The letter indicating the speed rating, S in this case, represents the maximum speed at which the tyre can carry a load corresponding to its load index.

NOTE: It’s important to comply with vehicle manufacturers’ load index and speed rating specs regardless of your actual speed, not only for safety’s sake but non-compliance makes insurance companies blind and deaf.

ROUND IT UP

To sum up, a 265/65R17 112S tyre has a tread width of 265 millimetres, a profile of 65 and is a radial tyre that fits on a 17-inch rim. The load index (112) indicates a maximum load capacity of 1120kg and a S speed rating certifies it up to 180kph.

 And then there’s this…

How to decipher the American Off-road Tyre Marking System

Most off-road tyres use the indexing format I’ve outlined above, but some use an older American marking system, for example 31×10.5R 15 LT.

• 31 = The outside diameter of the tyre in inches
• 10.5 = The section/tread width of the tyre in inches
• R = Radial Construction
• 15 = Rim diameter in inches
• LT = Light truck