Tyre size calculator for Australian cars, utes and 4WDs

This tyre size calculator compares your factory wheels and tyres with the ones you are considering, and shows what actually changes: overall diameter, speedo error, poke, track width, ride height and gearing. It then checks the whole package against VSB 14, ADR 18 and your state’s own rules, so you know before you buy whether the fitment is fine, needs engineering certification, or is not permitted at all.

How to read a tyre size

Take 225/45R17 94W. The 225 is the section width in millimetres, measured sidewall to sidewall, not the tread width. The 45 is the aspect ratio, so the sidewall height is 45% of 225, which is 101 mm. The R means radial. The 17 is the rim diameter in inches, because tyre sizing never fully went metric. Overall diameter is the rim plus two sidewalls: 17 x 25.4 + 2 x 101 = 634 mm.

The 94W on the end is the service description, and it is the part most people ignore. The 94 is the load index, worth 670 kg per tyre. The W is the speed symbol, good for 270 km/h. Both are load-bearing legally, and we will come back to them.

Four-wheel drive sizing often uses the older flotation format instead, like 33×12.50R15. That is overall diameter in inches, section width in inches, then rim diameter. A 33 inch tyre is 838 mm. LT before the size means light truck construction, and a C after it means commercial.

The sizes that matter are the ones on your tyre placard, usually inside the driver’s door jamb or behind the fuel flap. That placard, not whatever is bolted on now, is the legal reference point.

What the law actually says

There is a national code, and then there are eight states that each adopt it with their own additions.

The national document is Vehicle Standards Bulletin 14, the National Code of Practice for Light Vehicle Construction and Modification. Section LS covers tyres, rims, suspension and steering. Clause 1.1 lists the modifications you can perform without certification, and clause 4.2 sets the numbers. Alongside it sits Australian Design Rule 18, which governs speedometer accuracy.

Between them these two documents create three outcomes, and the tool above tells you which one you are looking at:

• Within the limits. No certification needed. Fit it.
• Outside the limits but certifiable. You need an engineering signatory, called VASS in Victoria, VSCCS in New South Wales, an Approved Person in Queensland and LVES in South Australia. Costs money and takes time, but it is a legitimate path.
• Not permitted. No signatory will certify it. A tyre below the placard load index sits here.

One thing worth being blunt about: the “stay within 3%” rule you will read on tyre retailer websites is an American rule of thumb. It appears nowhere in Australian law. Some of those same pages quote a 25 mm diameter allowance for passenger cars, which is also wrong. The figure in the code is 15 mm.

Diameter limits by vehicle type

Everything hinges on your ADR category, which is on the compliance plate or in the Register of Approved Vehicles at rav.infrastructure.gov.au.

Passenger cars (MA and MB) may run a tyre no more than 15 mm larger or 26 mm smaller in overall diameter than any tyre the manufacturer designated for that model. That is VSB 14 clause 4.2.4, and 15 mm is not much. It is roughly one step in aspect ratio.

Off-road four wheel drive passenger vehicles (MC) and goods vehicles (NA and NB), which covers utes and vans, get 50 mm larger or 26 mm smaller.

The trap is the middle ground. A RAV4, CX-5, Kluger, Outlander or X-Trail is almost always certified MA, not MC. They look like off-roaders and get the passenger car allowance. VSB 14 says so directly: soft-roaders “are not included”. If you are working from a 4WD forum thread about 50 mm, check your category first.

Two worked examples from the calculator above. A HiLux on factory 265/65R17 going to 285/70R17 gains 54.5 mm. That is 4.5 mm over the limit, which fails everywhere, and in New South Wales it lands on 7.02% against a 7% threshold. Drop to 275/70R17 and the gain is 40.5 mm, comfortably inside 50 mm. Four millimetres of tyre is the difference between a fitment nobody questions and one that needs an engineer.

Note also that the increase is measured against the largest size on the placard and the decrease against the smallest. If your model was sold with an optional larger wheel, that option is your baseline even if your car did not come with it. Queensland is explicit that the figure to use is the Design New Tyre Overall Diameter from the tyre standards manual, not the grown diameter a worn-in tyre reaches.

The speedo trap

This is the check that catches people who did everything else right.

Taller tyres cover more ground per revolution, so the speedometer reads lower than your true speed. For anything built from July 2006 as a new model, or July 2007 across the board, ADR 18/03 applies, and it is unforgiving in one direction: the indicated speed must never be less than the true speed. It may read high, by up to 10% plus 4 km/h, but it may never read low.

So the question is not “how much error do I have” but “does my factory margin cover the change”. Manufacturers deliberately calibrate speedometers to read high. UNSW’s Professor Michael Regan has put the typical figure at around 5%, though no government document specifies it and it varies by make. If your car over-reads by 3% and your new tyres are 6% taller, you are now reading below true speed and the speedo no longer complies.

Vehicles built before July 2006 fall under the earlier ADR 18 rules, which allowed plus or minus 10%. More headroom, both ways.

The fix is to measure rather than assume. Hold a steady indicated 100 km/h on a flat road on your current tyres and note what a GPS app says. That number goes in the optional section of the calculator and turns the speedo check from an estimate into a real verdict. If the answer is that you need correction, a speedo correction module or a gearbox drive gear change will do it, and in New South Wales it is worth knowing that affecting speedometer compliance is itself a certification trigger under VSI 06.

Offset, track, poke and why spacers are banned

Offset, stamped on the wheel as ET followed by a number, is the distance in millimetres from the wheel’s centreline to its hub mounting face. Lower or negative offset pushes the wheel outward. Higher offset tucks it in.

Track width changes by twice the offset change, because it happens at both ends of the axle. Drop from ET45 to ET30 and you have not gained 15 mm of track, you have gained 30 mm. VSB 14 clause 4.2.9 limits passenger cars to a 25 mm track increase, which works out to no more than 12.5 mm of offset change per side. Clause 4.2.11 gives off-road 4WDs and goods vehicles 50 mm. South Australia allows 26 mm on cars, with the rim offset not reduced by more than 13 mm.

Wheel spacers are not a workaround. Clause 4.2.2 prohibits spacers and adaptors between the wheel mounting face and the wheel unless they were fitted as original equipment. There is no thickness at which they become acceptable.

The other constraint is coverage. Clause 4.2.3 requires the wheel and tyre to be contained within the bodywork or mudguards, including legal flares, with the wheels pointing straight ahead. It also requires that no part of the wheel touches the body, chassis, steering, brakes or suspension under any operating condition, which means at full lock and full compression, not just parked on a flat driveway.

Stretched tyres and the JDM scene

Stretch is where a narrow tyre is mounted on a wide rim so the sidewall pulls in at an angle. It is a look, and it is one the code does not accommodate.

Clause 4.2.8 requires the rim width to be within the tyre manufacturer’s recommendation, and every tyre has a published approved rim width range. A 215/40R17 has a measuring rim around 7.5 inches and an approved range of roughly 7 to 8.5. On a 9.5 inch rim it is outside that range, and the calculator flags it. Beyond the paperwork, a stretched bead has less mechanical retention on the rim and can unseat under lateral load or on a kerb strike.

Run the stance example in the calculator and you will see it fails four separate checks at once: diameter 28.1 mm smaller against a 26 mm limit, track up 60 mm against 25, rim width outside the tyre’s range, and a load index of 83 on a car placarded at 91. The speedo is the only thing that passes, because smaller tyres make it over-read, which is the legal direction.

Load index and speed rating

Clause 4.2.12 is short and absolute. The replacement tyre must carry at least the lowest load rating on the tyre placard. Not close to it, not adequate for the car’s weight in your judgement. At least the placard figure.

This is where plus sizing quietly goes wrong. Drop from a 92 (630 kg) to a 91 (615 kg) and you have fitted a tyre rated to carry 15 kg less per corner than the car asks for, and it fails, even though the difference is invisible on the road and the tyre is physically the same size. It is also why you cannot borrow a size from a related model. A hotter version of your own car often runs a different placard, so the number on your door is the reference, not the one on a spec sheet. If the size you want is only made in a lower index, look for an XL or reinforced version of it.

Speed rating has a floor rather than a match requirement. Passenger vehicles need at least S (180 km/h) where the placard asks for more than S. Vehicles with special off-road features need at least N (140 km/h). Everything else needs at least 120 km/h. So a highway terrain tyre rated S on a 4WD placarded at S is fine, and dropping from a V to an S on a passenger car is permitted, but dropping to a Q is not.

Rim width for your tyre size

Every tyre size has a measuring rim, the width the manufacturer used to publish its dimensions, and an approved range either side of it. Fit outside that range and the published diameter and width no longer apply, which means the diameter figure you just checked against the law is no longer the diameter you have.

As a rough guide, a wider rim pulls the sidewalls upright, slightly increasing section width and slightly reducing overall diameter. A narrower rim does the opposite. The calculator gives an indicative range for the size you enter, but the tyre manufacturer’s own data sheet is the final word, and the good ones publish it.

Tyre size limits by state

Every state adopts VSB 14, but four of them add or alter numbers, and New South Wales replaces the diameter rule entirely with a percentage.

Two things that table cannot show. First, the lift allowances above 50 mm come with conditions: Victoria wants a commercial kit with a written statement, Queensland excludes body blocks entirely, and New South Wales requires certification for a body lift unless the kit is OEM-designed. Second, vehicles with electronic stability control attract additional scrutiny on suspension changes in several states, because ESC is calibrated to a known geometry.

Lowering has its own limits. VSB 14 allows a reduction of up to 50 mm or one third of the original suspension travel, whichever is less, and ground clearance must stay above 100 mm.

What happens if you get it wrong

Legally, a non-compliant fitment is a defect notice waiting to happen. Depending on the state that means a rectification notice, a formal defect, or an unregistered vehicle until it is fixed. It also surfaces at the worst time, which is the roadworthy inspection when you try to sell the car.

With your insurer, most comprehensive policies contain an exclusion for vehicles that are unroadworthy or modified outside legal requirements, and non-compliant wheels and tyres sit squarely in that description. Whether a specific claim is affected depends on the wording of your policy and the circumstances, so read your own product disclosure statement rather than assuming you are covered or assuming you are not. It is a conversation worth having with your insurer before you fit, not after.

Mechanically, the consequences are less dramatic than people claim but they are real. ABS and stability control are calibrated to a known rolling diameter, and changing it changes the wheel speed the system reads. Effective gearing shifts, so a 7% taller tyre is roughly a 7% longer final drive, which is noticeable on a loaded ute and dulls acceleration. Odometer error compounds quietly, so service intervals arrive later than the schedule intends. Wider tyres on lower offsets increase the scrub radius and put more load on wheel bearings and steering components.

Financially, the cost of getting it wrong is not the tyres, it is everything after them. A speedo correction module, an engineering certificate above the thresholds, and in the worst case a set of wheels that has to be sold on at a loss because the offset was never going to work. All of it avoidable by checking the numbers first, which takes about ninety seconds in the tool above.

Sources

Every threshold in this article and in the calculator comes from a primary document, current as at 5 September 2026:

• VSB 14 Section LS, National Code of Practice for Light Vehicle Construction and Modification, Department of Infrastructure
• ADR 18/03 Instrumentation, Federal Register of Legislation
• NSW Vehicle Standards Information 06 and 09, Transport for NSW
• VicRoads Vehicle Standards Information 8
• Queensland Department of Transport and Main Roads, Minor Modifications and Tyre Outer Diameter bulletins
• Department of Transport Western Australia, Information Bulletin IB-134
• SA.GOV.AU vehicle standards and modifications
• Tasmania Light Vehicle Inspection Manual, Reasons for Rejection
• Access Canberra modified light vehicles
• Northern Territory Inspection Manual for Light Vehicles

This article is general information, not legal or engineering advice. Rules change and inspectors have discretion. Confirm anything marginal with your state transport authority or an approved signatory before you fit.

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