Everything you need to know about buying tyres for your 4×4
Most 4×4 campfire discussions end up talking about tyres, here’s everything you need to know about buying tyres for your 4×4.
YOU CAN HAVE the best engine in the world, the best suspension, best everything but if your tyres are shot then it’s all for nothing. That’s because there’s just four little patches of rubber than connect your car to the ground, providing traction which translates into grip, handling, and ride.
And, for off-roaders we also need the tyre to be tough, resistant to punctures, and to handle a heavy load. Off-road requirements are hard to design for as the tyre must handle bitumen, dirt roads, rocks, sand, snow, mud and more… and in all sorts of temperatures.
Tyres are also consumables. You should never run tyres more than five years old, regardless of tread, because the natural oils leach out and the tyre loses flexibility and thus grip.
So tyres are important, expensive and frequently bought which is why there’s so much discussion about them. To choose the tyres you need, you first need to figure out what specification you require:
- Speed rating;
- Load rating;
- Construction; and
- Tread pattern.
The starting point for the tyre specifications is the tyre placard, which is typically found in the driver’s side door jamb. It will look like this:
Key parts of a tyre:
Speed Rating Explained
A single letter on a tyre is used to explain its speed rating, or how fast it can be driven. The law says that you must only fit tyres that have the same speed rating as the original tyres, but with off-road tyres that’s near impossible as the heavier, tougher tyres can’t handle the same speed as the lighter originals. Happily, there is an exemption for off-road vehicles which only need to be fitted with “N” rated tyres (140km/h) or better, and every off-road tyre should be able to meet that specification. Here’s a list of all the speed ratings:
|Speed rating||Max speed (km/h)|
Load Ratings Explained:
Tyres need to carry a load, and that’s the load rating, denoted by a number from 90 to 132. Any replacement tyre must be able to carry at least what the original tyres could carry. This is not a problem for off-road tyres, which are usually rated for higher loads than their stock equivalents. The table shows the load ratings:
Tyre Sizes Explained
Next you have to choose the correct size: overall diameter, width, and rim size. That’s denoted by numbers like “255/70/17” tyre, and this is what it means:
|255||Width of the tyre.||mm|
|70||Aspect ratio. 70 means that the height of the tyre is 70% of the width.||%|
|R||Radial. You won’t see much else, and it might be omitted.||N/A|
|17||Diameter of the rim, in inches. Yes, we’re mixing metric and imperial. No, I don’t know why.||Inches|
The theoretical overall diameter of the tyre in this case would be ((255 * 0.7) x 2 ) + (17 * 25.4) = 789mm or 31”. The exact diameter will vary from tyre to tyre, and also as the tread wears down.
You can fit larger tyres (width and/or height) but once you go above 5% greater height or 10% diameter you start to run into problems fitting the tyre onto the car, and these days changing tyre size from stock typically means you run afoul of road regulations. What you can do is change rim size, for example a 255/75/16 tyre is the same diameter and width as a 255/70/17, as is a 255/65/18. In general, go for the smallest rim diameter you can to maximise the height of the sidewall so you can deflate the tyres for better off-road performance.
There are two basic types – P for Passenger, and LT for Light Truck. You want LTs for off-roading because they are stronger, but your new 4×4 almost certainly has P-construction tyres because they’re lighter and handle better. Ignore P-XL or other reinforced variants of Passenger construction unless there is no option, such as smaller tyres for some off-roaders.
Tread Pattern Explained
There are three basic choices; road, all-terrain and mud tyre. The difference is the size of the tread blocks and spacing between them which become larger from road to mud, and that improves offroad performance at the expense of onroad handling, ride, noise and fuel consumption.
On occasional dirt roads: The stock tyres will be mostly fine.
Mostly on dirt roads: Here’s where you need to consider an upgrade to light-truck construction tyre for strength to deal with the stones, undulations and general abuse tyres get on dirt roads. The tread pattern is less important, but a good all-terrain would definitely offer better handling and grip than a road pattern tyre. You don’t need a mud terrain.
Off-road – mud, hills, rocks and the like: Again you must have a light-truck construction tyre, and here the tread pattern becomes more important in proportion to the time and difficulty of the offroad drive. A modern vehicle on modern all-terrain tyres does very well compared to an older car on mud tyres, so for this terrain all-terrains should be your minimum with a good look at muds if you won’t spend most of your time on-road, and you won’t be doing many road kilometres which will just wear the muddies out for no reason.
The Big Lap, remote touring: Light-truck is a given here, and at least an all-terrain pattern as you’ll need to move from many long stretches of bitumen to dirt roads to offroad as you move around. Take a second spare tyre for these trips.
|Rim||The metal wheel on which the tyre is fitted|
|Bead||The part of the tyre that seals against the rim|
|Sidewall||The vertical side of the tyre, from bead to shoulder|
|Tread||Part of the tyre that touches the road|
|Tread block||A single block of rubber, many of which together form the tread|
|Shoulder||Part of the tyre between the tread and the sidewall|
What about spare tyres?
You must have a full-sized spare for any off-road or remote touring work, not one of those temporary-use spares known as space-savers or TUSTs (Temporary Use Spare Tyres) because TUSTs are only designed to be used for about 80km, at a speed of no more than 80km/h. That doesn’t include off-road work when heavily loaded, or towing.
Many off-roaders have a full-sized spare, but if yours doesn’t you might be able to buy one – check eBay or similar before buying one off a dealer. But first, check that your car can actually take the full-sized spare as the spare compartment may not actually be able to hold a full-sized spare tyre. In that case, you’re typically looking at a roofrack, or perhaps a spare wheel carrier to go on the back of the car.
Some vehicles come with a full-sized spare tyre on a steel, not alloy rim. That will work fine, but just not look as good.
Top Tyre Tips:
- Rotation: Tyres should be rotated every 10,000km. This means swapping front left to rear right, rear right to rear left, rear left to front right and front right to front left – or some other combination. The spare may be included in the rotation pattern which means the tyres last 1/5th longer, but I don’t do this so that if I need to use the spare it is not worn at all and therefore the chances of a puncture are reduced;
- Inner tubes: a tubeless tyre is not designed for an innertube and will not benefit from one. The innertube may damage the inside of the tyre which has a membrane to seal the air in, and it will generate heat that is not good for the tyre. However, for short-term use you do what you have to.
- When to replace tyres: the legal limit is 1.5mm of tread depth. This is designed for road tyres which are typically 3 or 4 mm tread depth. Off-road tyres may be around 20mm tread depth, and stop working effectively near 5 or 6mm. All tyres should be replaced after 5 or 6 years of age regardless of tread condition.
- Tyre pressure: drivers familiar with other motor recreations have a hard time understanding offroaders lower their tyre pressures for off-roading. This is to reduce the bouncing on rough ground, increase contact patch which means the tyres “float” more over soft ground and that reduces rolling resistance as well as increases tread area, and finally it reduces the chances of a puncture. However, the lower pressure means you can’t drive as fast due to heat buildup in the tyres.
- Directionals: some tyres are designed only to work in one rolling direction so cannot be swapped left/right unless the tyre is removed from the rim and turned around. These are good for high-performance work, but not for touring because of lack of flexibility.
- Asymmetric tyres: these are tyres that don’t have the same pattern on the left of the tyre as the right. They can be swapped left to right.
- Alignment: the wheels on a car may not be designed to point exactly straight ahead, or vertically upright! The differences are small but important, and maintain them is part of tyre management for best life and performance. Get an alignment done every 10,000km, with every new set of tyres, and after suspension changes.
Are stock tyres any good?
Two answers – no, and perhaps. The more utilitarian vehicles such as the utes have tyres that will be all right for some dirt driving and basic off-roading, but some others will have tyres that you’ll need to change if you try anything more adventurous than your driveway.
In particular, premium model with thin, low-profile tyres are the ones to change over as soon as you can, and flick the original on eBay. If you can, also change the rims to a smaller size such as maybe 17-inch rather than 18-inch or 19-inch, so you get more rubber between the rim and the road.