Upsizing Your Tyres
Written by Dan Everett
Get any group of 4WDers together and sooner or later the topic of tyres will come up. “How big are they?” It’s almost an unspoken rule of 4WDing that you’re judged by either the distance you’ve travelled, or the size of your tyres. It makes sense too; chest beating aside, tyres are the key component between you and the terrain you’re driving. If you had a 1000hp engine and the world’s flashest camper you’re not getting anywhere without the right rubber putting power to the ground.
A good set of tyres can make your 4WD go further, use less fuel, float across surf-soaked sand and even tow better at freeway speeds. The wrong set can turn even the best tow-tug setup into an unusable waste of driveway space. So, keen to know what the key is to tyre selection and stepping up a shoe-size in your 4WD? You’re in luck, because we’ve already made all the mistakes for you.
If you’re the type who has driven 500,000km offroad on stock size rubber you might be scratching your head wondering why people are so emotional about big tyres. That’s fair enough, you’re clearly performing miracles. Regardless, what is it that draws people to them? The short answer is offroad ability. But it might not be in the way you think.
There’s a common misconception that the greatest benefit in airing down your tyres is the increased width you gain as they bag. The real benefit is in the length of the tread touching the ground. The more tread on the ground, the more grip you’ve got, and the less likely you’ll get bogged. Larger diameter tyres have a larger footprint. It not only gives them more traction but also makes them less likely to dig down into soft terrain like mud or sand. Think of it like standing on two feet instead of one and you’re on the right path.
Of course, the benefits don’t end there. The other factor is the diameter itself. Sure, it might seem reasonably obvious, but the benefits of a larger rolling diameter are more than you’d expect. The big one often touted is more clearance under the diffs, but I can count on one finger the times I’ve been diffed out, and that was in a HiLux on 37in tyres in a situation that would’ve required a lot of quick explaining if the long arm of the law had found us. The real benefit is improved angles. The larger diameter effectively makes every obstacle you come across smaller by the same amount. It’s like running over a small piece of wood on a skateboard or a mountain bike. The large the tyre, the less it gets knocked around by obstacles, and that includes corrugations.
It's not all just about offroad ability either. More aggressive tyres are tougher offroad too. Despite stepping up to a reasonably mild 285/70 R17 on the Ranger compared to the stock 265/65 R17, the actual tyres carcass is now protected by an extra 10mm of rubber thanks to the more aggressive tread pattern. The thick rubber wraps around onto the sidewalls providing more protection against staked tyres, and the tyre itself is more difficult to penetrate thanks to a beefier construction, although more on that later.
“Yes!” I hear you say, “all tourers should run 40s”. Eh, not exactly. Y’see, as much as I’m a proponent for larger tyres they’re not without their fair share of setbacks. Throughout your drivetrain you’ll find gear ratios all calculated to work together. X amount of RPMS in the engine, multiplied through Y ratio in the gearbox then Z through the diffs, and it’s all worked out on the stock diameter tyres. Changing one without changing another to suit can knock your gearing around. Now it’s not all bad news. Some 4WDs from stock struggle to hold a gear with a camper on the back at set cruising speeds, they’re on the edge between one gear and another. Fitting larger tyres can often knock things around enough that you’re firmly in one gear to do the same speed letting the tow-tug power through easier. The result is less strain on your drivetrain and a saving at the pump as the engine is no longer labouring. It sounds backwards, but the science checks out.
The other main issue is unless you’re excited at the idea of taking to your pride and joy with a 5in grinder, making even moderate-sized tyres can be more trouble than it’s worth. Your options generally fall into monster lift kits, which open up their own can of worms, or make the tyre fit at stock height (even with a mild lift kit you’ll quickly bump back to stock height through difficult terrain). It’s a delicate juggling act to make the tyre fit between the front bar and firewall, and if you’re anything like me will see you taking to a new 4WD with a lump hammer.
There’s no such thing as a free lunch and that extends right through to tyres too. You don’t get physically larger, thicker, and stronger tyres without having a few drawbacks. The main issue is increased load on your drivetrain. Heavier wheels spinning with more momentum will put more load on drivetrain components like CV joints, driveshafts, brakes, transfer cases, and gearboxes. There’s no dodging this one. Breakages are more likely, acceleration times will slow, and braking distances will increase. How much depends on how big a jump you make in tyre size, although different brands of tyres have different weights so it’s worth taking that into consideration.
Now as much as we like to pretend they don’t exist, the boys in blue are privy to this info as well so in most states anything over a 50mm increase in rolling diameter will require engineering; a complicated process that requires all sorts of nonsense like swerve tests and brake tests. Actually, that stuff might be reasonable, but it’ll still hit your hip pocket.
If you’re travelling in remote country you’ll need to factor in the availability of spares as well. That super large sidewall tyre might look the goods on your rig and give you 17.376 percent more capability offroad, but if you’re sitting around with your thumb up your backside waiting for spares you won’t be bragging to too many people about it.
So how do you go choosing the right set of shoes for your rig? It can be broken down into a couple of categories.
The first is the tread pattern. Larger tyres alone won’t give a noticeable amount of grip if they don’t have the teeth to bite in. In the early years the general rule was the larger and deeper the voids in the tread the more grip the tyre would have, sort of like a tractor tyre. The problem with these tyres are the lugs could physically deform under load making the 4WD feel unsettled cornering, and would also perform poorly on side slopes. These days with computer-modelled patterns and intelligent rubber compounds you’re able to get more grip out of a less aggressive tread pattern. After all, there’s a reason many tyre tread patterns are starting to look similar. If you’re spending your weekends bouncing off the limiter in Victorian mud, a full-blown mud tyre might still be the key to success, but for general offroading including a fair chunk of low-range action, a milder mud-tyre or aggressive all-terrain will be a better balance between offroad ability and on-road manners. If you’re more of a tourer than an offroader, a traditional all-terrain will give most of the benefits of an offroad tyre while still driving like a highway-terrain.
You’ll also want to keep an eye out for a big LT stamped into the tyre’s body. That stands for Light Truck construction. If you’re hauling concrete slabs the difference is a higher load rating, but for us offroaders LT construction means the tyre will have a physically stronger bead so it’s capable of running lower pressures, will have thicker plies inside the tyre to protect against stakes, and will generally have deeper tread too.
People often say you can’t fit a square peg in a round hole. But they’re wrong. Fundamentally so. Context is important, and just like a 1cm square peg will easily fit in a 10cm round hole, fitting the right tyres to your 4WD isn’t all about the tyres. For a lot of people, the wheels of choice are just the right style, but there’s a few things you need to consider.
Most modern 4WDs (and even some 20yo ones) run larger brakes. That almost always counts out smaller rims like 15s and some 16s. Alloy wheels are generally thicker than steel to account for material strength so they’ll run into brake calliper clearance issues before their steel counterparts.
The offset also comes into play. Put simply, it’s the relationship between the mounting face of the wheel, and where the tyre mounts. A +20 offset means the tyre is kicked in towards the vehicle 20mm from the centre line. A -44 means the tyre is kicked out 44mm from the centre line. Getting the balance right is key, which is why many people leave it to the wheel and tyre shop. Too far out and you’ll have clearance issues with the guard, too far in and you’ll have clearance issues on the firewall and steering arms. Each vehicle is different, so there’s no set formula on this one.
While cheap wheels with sweet financing deals might seem appealing it’s worth noting any imperfections in the wheel are amplified the larger the tyre choice. Things like inconsistent metallurgy or even deformed wheels will all become bigger issues, with larger tyres requiring huge amounts of weights to try and keep steering vibrations down.
The other issue to consider is the diameter of the wheel itself. In years gone by, the rule was the more sidewall the better. Here, in reality- land, it’s pretty clear you’re not jumping off three story buildings, so larger rims with lower profile tyres generally give more steering feedback and more direct handling without compromising offroad ability.