Can I fit bigger tyres to my 4WD?

Like the shoes you wear, upgrading the wheels and tyres on your car is a simple way to dress up its appearance and improve vehicle dynamics.

Without any laws preventing you from sliding down the red carpet wearing a pair of Yeezys, strolling along the beach in Gucci loafers or flip-flopping to the shops in your Havanas, only the fashion police can pull you up for going too far.

But the real ‘boys in blue’ can write you a ticket – or worse, slap you with an unroadworthy and impound the car – if you don’t follow some clear rules on vehicle modifications when fitting different wheels and tyres.

Over time, wheel sizes in passenger cars have increased dramatically, with some high-performance vehicles now fitted as standard with rims as large as 22 inches in diameter. As a consequence, tyres have become wider and with lower-profile side walls.

With bigger wheels becoming the norm rather than the exception these days, there is a temptation for those who want to express themselves more overtly to fit even bigger aftermarket wheel options, in some cases as large as 26 inches in diameter.

Similarly, the significant rise in popularity of recreational off-roading has been a boon for four-wheel drive modifications and the fitment of specialised 4x4 wheels and tyres which buck the road-going trend: smaller rims wearing ballooning rubber better suited for off-road use.

So, if you’re looking to fit some new boots to your tourer or tow vehicle, let’s find out how far can you go when fitting different wheels and tyres.

Check the rule book first

First of all, there are national guidelines to vehicle modifications that are created by the federal Department of Infrastructure, which manages the certification process for vehicle use in Australia, and in accordance with Australian Design Rules standards.

As outlined in the Vehicles Standard Bulletin 14 – the National Code of Practice for Light Vehicle Construction and Modification – there are unique rules for both passenger vehicles and four-wheel drives which we will explain separately.

In all cases though, replacement tyres have to be legal for road use (that is, not racing slicks), made using the same construction as the originals (ie. radial tyres) and the load rating is not lower than the minimum load rating prescribed by the manufacturer and listed on the tyre placard.

Checklist for fitting upsized wheels and tyres

• Tyres must be legal for road use
• Tyres must match or exceed the speed and load rating for the vehicle
• Wheels must meet Australian Design Rule standards
• Wheels must have identical stud/bolt pattern
• Tyres must remain inside the bodywork when the steering is pointed straight ahead
• Tyres must not contact any part of the bodywork, steering, braking or suspension components
• Rear tyres can only vary in aspect ratio by 10 per cent to the front tyres
• For passenger cars, wheel track can be no wider than 25mm more than original specification
• For passenger cars, the difference in rolling diameter of the wheels and tyres must not be greater than 15mm or less than 26mm from the original specification
• For passenger cars, wheels can be no wider than 30 per cent from original specification
• For 4WDs, the difference in rolling diameter of the wheels and tyres must not be greater than 50mm or less than 26mm from the original specification
• For 4WDs, wheels can be no wider than 50 per cent of the original specification
• For 4WDs, wheel track can be no wider than 50mm more than the original specification

Passenger cars

For passenger cars specifically, when replacing the tyres – even if the wheels are the same size as standard – the speed rating of the tyre has to be a minimum of at least an S designation (suitable for a minimum top speed of 180km/h).

In special circumstances, such as vintage cars, the speed rating may be less than that only if the tyre is rated higher than the vehicle’s maximum speed. This does, however, require permission from your state’s registration authority.

When fitting new rims – irrespective of their size – the combination of the wheel and tyre must, firstly, comply with Australian Design Rules standards for road use and not feature a full circumferential weld other than one that attaches the wheel centre to the rim itself.

The wheels must be designed for the same stub axle or hub assembly with an identical stud/bolt pattern, pitch circle diameter and location method. They must be located by the same diameter centre spigot and only use suitable adaptor rings where necessary.

The wheel nuts or bolts must have a thread engagement length that is at least equal to the thread diameter and wheel spacers must not be used unless fitted as original equipment by the manufacturer.

The wheels will be deemed illegal without a separate engineering certificate if the brake callipers, hubs, suspension and steering components need to be modified in any way.

Similarly, no part of the wheel or tyre must touch any part of the body, chassis, steering, brakes or suspension under any operating condition and the wheels must be contained within the bodywork (including any flares) when the wheels are pointed in the straight-ahead position.

When upsizing the wheels and tyres, the overall diameter must be no more than 15mm larger or 26mm smaller than the tyre size designated by the manufacturer for that particular model.

Changing the rolling diameter affects the accuracy of the speedometer – larger wheels will make the speedo read slower than your actual road speed, which increases the risk for being booked for speeding, while smaller wheels will do the opposite – and there’s a legal requirement for the wheel and tyre combination to maintain the degree of accuracy to within ADR 18/03.

If the wheels are wider, they must not be more than 30 per cent wider than the largest optional tyre fitted by the manufacturer and, as before, must be contained within the bodywork of the vehicle.

And, if you are varying the aspect ratio (the relationship between the height of the sidewall compared to the width of the tread) of the tyres between the front and rear axles (ie. fitting wider rear tyres with a lower profile), the variance must not be greater than 10 per cent.

As an example, having a 235/45 front tyre and a 275/35 rear tyre is allowed whereas fitting a 295/25 rear tyre would be illegal.

Finally, the consequences of fitting different wheels and tyres must not increase the overall track width of the vehicle (the distance across the car from the centre of each tyre) by more than 25mm beyond the maximum specified by the vehicle manufacturer.

This means that, essentially, the wheel rim offset (the distance between the centre line of the rim and the face of the hub) cannot be more than 12.5mm greater than the original.

Technically, if you are fitting a high-performance wheel and tyre package to a standard vehicle it is also a requirement to fit the matching suspension components – springs, shock absorbers and anti-roll bars – from the manufacturer.

Four-wheel drives

Fundamentally, the same principles apply when fitting specialist 4x4 off-road wheels and tyres in maintaining the integrity of the vehicle’s safety.

But there are some rules that are unique to dedicated 4WDs (not SUVs, which are deemed passenger cars).

Firstly, the minimum speed rating for off-road tyres is N (with a minimum top speed of 140km/h). But check your vehicle’s tyre placard for the manufacturer’s minimum speed rating first as most modern 4x4 off-roaders are more capable than that.

Secondly, the tyres must not be more than 50 per cent wider than the manufacturer’s widest option, the rims must not exceed the recommendations for the correct fitment of the tyre and the wheel track must not be greater than 50mm wider while ensuring the wheels remain inside the bodywork and maintain clearance with all components and panels.

And finally, there’s a bit more grace when increasing the rolling diameter for off-road tyres, which must be no more than 50mm larger or 26mm smaller than the original fitment tyre.

Either way, the same rule applies that the difference must keep the degree of accuracy for the speedo within the ADR 18/03 guidelines.

A larger rolling diameter naturally increases the vehicle’s ground clearance. And, if you are considering adding a lift kit for even greater off-road abilities, you can raise the vehicle by no more than 50mm without additional certification if the suspension maintains at least two thirds of the original rebound travel and there is no variation in the wheel track width.

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