Bigger tyres, bigger problems? Understanding tyre regulations

Bigger tyres can mean bigger issues! Stay compliant with our guide to tyre regulations, covering legal limits, sizing, load ratings, and more.

If you’re reading this, you’ll almost certainly want to replace the stock tyres on your 4×4. The reason is that the maker of your 4×4 does not know for what purpose you bought it, and the vehicle has to pass strict laws on emissions and noise.  The manufacturer is also quite interested in profit.  So for all these reasons, they fit general-purpose tyres. 

But your purpose is not general, it is offroading, and you are not bound by the regulations they are. So, you need a tyre designed and optimised for your purpose, which is offroad use. Typically, this tyre will be taller and of a different, tougher construction with a more open tread pattern.  That’s a different tyre from the standard, and therefore, it is a modification. This means you must fit the tyre within the limits of the regulations.

Where do you turn for advice on tyre regulation?

So, where do you turn for advice?  Your tyre shop?  Possibly, there are some good ones.  There are also shops that don’t understand load ratings, inflate Jimny tyres to 35psi and state that the only legal tyres are those mentioned on the placard.  Sorry tyre industry, but you have proven that your fitting shops are not reliable sources of information, however much you promote your knowledge and professionalism.

You can find all the tyre regulations on your state government road authority website. The tricky thing about the tyre regs is that there’s not just one, there are several, and the ones you need to comply with don’t always specifically relate to tyres. It’s complex, so let’s break it down.

Diameter

The first limit is the diameter. The additional diameter permitted varies depending on class of vehicle, but for 4x4s it’s typically plus 50mm, for cars, 15mm.  The definition of a 4WD? Here are the words from Vehicle Standards Bulletin (VSB) 14, Section LS: “4WD passenger vehicles specifically designed for off-road use”.  That doesn’t include utes, but they’re actually allowed such mods anyway as they’re in a commercial vehicle classification, the N category of vehicles as opposed to passenger cars which are M category.

There are plenty of tyre diameter calculators on the web, so pick one. They will work out the diameter based on the tyre specs, such as 265/65/17. Read our previous article for an explanation of those numbers.

However, larger tyres cannot foul any part of the body when the suspension flexes and/or the steering wheels are turned, so that needs to be checked. It’s at this point that sometimes, you find two tyres of identical sizes are not, in fact, exactly the same diameter or width as one fouls, and one does not. Here are the words from VSB14: “No part of the wheel must touch any part of the body, chassis, steering, braking system or suspension under any operating condition.”

The 75mm rule

There is also the “75mm rule”, which means you can lift a 4WD by 75mm with a combination of 50mm suspension and 50mm tyre.  But 50 plus 50 is 100, right?  Not in this case, as a 50mm tyre diameter increase means a 25mm increase in height, so it’s 50+25=75mm.  The existence of this 75mm lift rule is a maximum, and like all other limits, if another rule kicks in first, then that’s as far as you can go.

Width

Then we come to width. I did say in previous articles not to bother going wider. But often, more height means more width, so you end up wider anyway.  The limit for width for 4WD is 1.5x the widest original tyre as per the placard.  But that’s a perfect example of an indirect regulation, as chances are you’ll run afoul of other limits first. 

For example, there are limits on how much of a track increase you can have, which is the width between wheel centres on an axle, and that dictates the maximum offset you have chance (distance from wheel hub to centre of wheel).  That, in turn, starts to limit the width of the rim, which limits the width of the tyre as every tyre has a maximum and minimum rim width it can be fitted to.  We’re dealing with tyres here, not rims though. That’s a related but separate subject.

Load rating

Then we come to load and speed rating.  Load rating, which we covered in a previous article, is easy.  You find it on the placard, and any tyre you fit must meet or exceed that load rating, which is never a problem as any 4×4 tyre is going to be heavy-duty. A note about GVM upgrades, which involve an axle re-rating, though: “Where a vehicle has its GVM re-rated, the tyre load capacity must be capable of carrying the revised GVM, both in total and across individual axles.”

Speed rating

Next is the speed rating, and here we have a problem.  The favoured 4×4 tyre will be heavy-duty, light-truck construction with deep tread blocks and an open tread pattern.  This does not make for a high-speed tyre, so it is entirely possible that such tyres may have a lower speed rating than the placard. 

Happily, VSB14 makes provision for this, saying that if a vehicle has “special features for off-road use” then it need only fit N, or 140km/h rated tyres, regardless of the placard. And just in case it wasn’t clear, VSB14 also says the tyres must be “rated by the tyre manufacturer as being suitable for road use”.

Replacing tyres

Ideally, you want to replace five tyres at a time, yes, including the spare, so it’s matched and doesn’t get too old.  Especially with AWD 4X4s, which are those which are constant-4X4, you want to replace at least four tyres at a time.  The reason is that you’ve got three differentials to consider: front axle, rear axle and centre.  These differentials have two output shafts and complex gears, which don’t do anything until there is a difference in speed between the two shafts, such as when you’re turning.

Another reason for shaft speed is tyre diameter differences, such as when you run a new tyre and an old tyre on the same axle.  The difference in diameter might be 2-5% just due to worn tread blocks, and that then means the differential will be doing things it was never designed to do, and will fail early.  So, you want to keep all four tyres with similar tread levels over the life of the car. 

For part-time 4WDs, those that run 2WD on the road, this is less important as there is no differential between front and rear axles, and when the car is in 4WD, there will be little enough traction such that a bit of tyre diameter difference doesn’t matter much.

So there you have it

So there you have it.  You need to ensure the diameter increase and width increase are within limits, the load rating is at least that of the placard, and the speed rating is at least N.  Read your state’s regulations, which probably refer to VSB 14, Section LS, then take that knowledge with you when shopping.  Enjoy your new rubber!

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