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Understanding Tyre Specs

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We’ve all seen the big bunch of jumbled letters and numbers plastered along the side of each and every road rated tyre, right? While we all know these are the tyres specifications, understanding what it all means can be a bit like deciphering a top secret code straight from NASA if you’re not a fully fledged expert. The good news is if you’ve ever wanted to learn the lingo, you’re about to receive a crash course! So roll up your sleeves and button up the cuffs, it’s time to get stuck into it…

DECIPHERING THE CODE

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The specific details of a particular tyre are usually displayed in one of three ways on the tyres sidewall. Here’s how to break it all down.

275/75R16 100T

275 = section width (in millimetres)
75 = sidewall aspect ratio (as a percentage)
R = tyre construction (in this case it’s a radial)
16 = rim diameter (in inches)
100 = load rating
T = speed rating

33×12.50R15LT
33 = diameter of tyre mounted on rim (in inches)
12.50 = section width (in inches)
R = tyre construction (radial)
15 = rim diameter (in inches)
LT = Light Truck construction

9.50R16LT
9.50 = width (in inches)
R = tyre construction (radial)
16 = rim diameter (in inches)
LT = Light Truck construction

ASPECT RATIO – WIDTH TO HEIGHT

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Put simply, the aspect ratio is used to determine the size of a tyres sidewall. To do the calculation, you’ll need to breakdown the tyre size indication numbers.

As mentioned above, the first of the numbers indicates the tyres width in millimetres. The second number specifies the size of the sidewall, which is indicated as a percentage of the tyres width. This is typically known as the aspect ratio or profile. So if you had a 285/75R16 it means the tyre is 285mm wide, and the sidewall would be 75% of 285mm, which amounts to 213.75mm tall.

LOAD INDEX (Load Range)

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The next set of numbers you’ll see is the “service description’, which indicates the tyres load index and speed rating.

The tyres “load index” relates to its maximum carrying capacity, although you’ll need to cross reference the number to a load index chart to determine the maximum load capacity per tyre, and at what tyre inflation pressure. It can also be indicated as a “Load Range” with a single capital letter.

When choosing a new tyre, the load index must meet or exceed the value displayed on the vehicles placard to ensure it’s up to the task.

LOAD INDEX CHART

Load index Load in Kg per tyre Load index Load in Kg per tyre Load index Load in Kg per tyre Load index Load in Kg per tyre Load index Load in Kg per tyre
62 265 75 387 88 560 101 825 114 1180
63 272 76 400 89 580 102 850 115 1215
64 280 77 412 90 600 103 875 116 1250
65 290 78 425 91 615 104 900 117 1285
66 300 79 437 92 630 105 925 118 1320
67 307 80 450 93 650 106 950 119 1360
68 315 81 462 94 670 107 975 120 1400
69 325 82 475 95 690 108 1000 121 1450
70 335 83 487 96 710 109 1030 122 1500
71 345 84 500 97 730 110 1060 123 1550
72 355 85 515 98 750 111 1090 124 1600
73 365 86 530 99 775 112 1120 125 1650
74 375 87 545 100 800 113 1150 126 1700

SPEED RATING

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The letter following this number is the speed rating, which is indicated as a speed category symbol to be cross referenced with the appropriate chart. It determines the maximum speed at which the tyre can carry a load corresponding to its load index. So for example, ‘S’ would refer to a maximum speed of 180km/hour.

SPEED SYMBOL CHART

Speed Symbol J K L M N P Q R S T U H V W Y VR ZR
Speed in (km/per hour) 100
110
120
130
140
150
160
170
180
190
200
210
240
270
300
>210 >240

Tyre Age

Some car makers advise against using tyres over six years old. Even without use heat, sunlight and ozone can affect the tyre compound causing it to become brittle. To check the age of your tyres, look at the Tyre Identification Number (TIN). The last four digits indicate the week and year the tyre was made, for example a tyre with TIN XXX4804 was manufactured in the 48th week of 2004.

 

WHAT’S A PLY RATING?

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The ply rating gives you a good indication as to how strong the tyre’s carcass is

Originally ply ratings referred to the number of cotton layers used to strengthen a tyre’s casing, which in turn indicated the tyres strength and load carrying capacity. These days, steel and radial construction has pretty much replaced cotton, so although a tyre might indicate a ply rating like say “10 Ply”, it doesn’t necessarily mean it’s constructed of 10 individual layers of cotton. It should however offer the equivalent in strength. So in essence it’s a strength comparison based on previous standards for bias ply tires. With steel ply and radial construction replacing cotton, the “Load Range” is the new distinction, but for us travellers a high ply rating is a great way to gain a little confidence in the tyres overall strength.

TYPES OF TYRES

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Mud terrain tyres are typically of the tougher Light-Truck construction with an aggressive tread pattern for superior grip off the beaten track

First things first, there are a few main categories that tyres get grouped into, which are indicated as large capital letters on the sidewall of the tyre. These indicate the terrain the tyre has been purposely designed to excel in.  Here’s a list of the common ones you’re likely to see.

H/T (Highway Terrain): This type of tyre is typically designed to stick to the pavement. Being specifically designed for this task, they generally last longer, provide better grip, and produce less noise than other types of tyres travelling on tar roads.

A/T (All Terrain): Think of it as a cross between an on and off road tyre. You’ll find the tread is a bit chunkier than an on road Highway Terrain tyre, and a compromise between the two.

M/T (Mud Terrain): These are usually tougher in construction with a much more aggressive tread pattern. It’s usually designed with durability and off-road grip in mind, the down fall is on road performance is usually compromised, with things like poor wet weather grip (on tar), increased road noise and reduced tread life being the trade off.

M/S (mud and snow): The ‘M+S’ marking usually indicates at least 25 per cent of the tread area is an open tread pattern. For tyres to be marked as Mud and Snow, they must have outer tread grooves that lead into the centre of the tread.

You might also spot an “R” at the end of indications, which simply stands for radial construction. So “ATR” would indicate an All Terrain tyre of radial construction.

TYRE CONSTRUCTION

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The tyres overall construction is also specified using letters.  For example, the letter “P” indicates it’s a P-metric Passenger Car tyre, the letters “LT” stand for Light Truck and a “T” means it’s for Temporary use.

Now each style of tyre is designed with different thresholds in mind when it comes to load carrying capabilities, tyre inflation and speed ratings just to name a few. Put simply, using the wrong tyre for the job can be extremely dangerous.

LIGHT TRUCK (LT) –Typically built with load carrying capacity and strength in mind, they have a stronger carcass to handle the load requirements of light trucks. Most vans of ute’s will usually specify these types of tyres, and you’ll find the majority of All Terrain and Mud Terrain tyres are rated as the tougher Light Truck (LT) construction too.  

PASSENGER (P) –Passenger-Rated tyres have a higher speed rating and lower load rating. they are of a lighter construction than Light Truck tyres and are typically used with Highway Terrain tyres,

TEMPORARY USE (T) – The name says it all, eh? These tyres usually carry a very low speed and/or load rating, and are typically used as a spare tyre in emergency situations.  

RADIAL CONSTRUCTION

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The big “R” stamped next to the tyre size indicates that the tyre is of radial construction, which accounts for the vast majority of tyres manufactured today thanks to its superior performance over the older Cross-Ply tyre design. Cross-Ply tyre specs simply skip the “R” on the side wall.   

MAXIMUM INFLATION PRESSURES

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Each tyres maximum inflation pressure (when cold) is written in the fine print on the sidewall. This is commonly interpreted as the recommended inflation pressure, but put simply – it’s not! This specification is literally designed to state the maximum inflation pressure the tyre is designed to contain. This is always checked when the tyre is cold, so before driving, during cold conditions or before the day’s ambient temperature can cause the tyre pressure to temporarily increase.    

MEET THE AUTHORBorgy

Michael “Borgy” Borg

Borgy’s one of those blokes who lives and breathes offroad adventure. He’s travelled to almost every extremity of the Australian continent, built 4WDs and camper trailers from the ground up and tackled some of the most epic adventures Australia has to offer.

Being a mechanic by trade, he’s customising both of his Toyota LandCruisers, ‘Toot’ the Troop Carrier and ‘Uncle Grump’, his big red 80 Series Cruiser. With plenty of tough low range kays under his belt, you can bet your bottom dollar he’s learnt the art of bush mechanic fixes. In fact, Borgy reckons relaxing around the campfire after an epic day on the tracks is what 4WDing is all about, not to mention that feeling of freedom you get when you lock in the hubs!

Pics by Matt Fehlberg

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