Free Australian tyre size comparison · No sign-up · Runs entirely in your browser
Compare two tyre sizes side by side — sidewall height, overall diameter, circumference, revolutions per kilometre and the speedometer error a size change introduces.
Enter tyre size (e.g. 225/45R17 from sidewall of tyre)
Comparison tyre
| Metric | Tyre 1 | Tyre 2 | Diff |
|---|
Format: 225/45R17 — 225 = width in mm; 45 = aspect ratio (sidewall height as % of width); R = radial; 17 = rim diameter in inches.
Tyres also show a load index (e.g. 91 = 615kg per tyre) and speed rating (e.g. V = up to 240 km/h). Always match or exceed the manufacturer's recommended ratings listed in your owner's manual or door placard.
Every tyre carries its dimensions on the sidewall in a standard sequence. Taking 225/45R17 91V apart:
| Element | Means | In this example |
|---|---|---|
| 225 | Section width in millimetres, tread wall to tread wall | 225 mm wide |
| 45 | Aspect ratio — sidewall height as a percentage of width | 45% of 225 = 101.3 mm |
| R | Construction: R for radial, the modern standard | Radial |
| 17 | Rim diameter in inches | 17 in = 431.8 mm |
| 91 | Load index — a coded maximum load per tyre | 91 = 615 kg |
| V | Speed rating — maximum rated speed | V = 240 km/h |
The size code describes three physical dimensions. Seeing where they sit on the wheel makes the arithmetic obvious:
The key thing the diagram shows is that there are two sidewalls in every diameter — one above the rim and one below it. That is why the formula doubles the sidewall height before adding the rim. Raise the aspect ratio by ten points on a 225-wide tyre and you add 22.5 mm of sidewall, but 45 mm of overall diameter.
| Variable | Why it matters |
|---|---|
| Width | Contact patch and, indirectly, grip and rolling resistance |
| Aspect ratio | Sidewall height — the main influence on ride comfort and steering feel |
| Rim diameter | Converted from inches at 25.4 mm per inch |
| × 2 on sidewall | There is a sidewall above and below the rim |
| Revolutions per km | What your speedometer and odometer actually count |
| Step | Calculation | Result |
|---|---|---|
| Sidewall height | 225 × (45 ÷ 100) | 101.3 mm |
| Rim in millimetres | 17 × 25.4 | 431.8 mm |
| Overall diameter | 431.8 + (2 × 101.3) | 634.3 mm |
| Circumference | 634.3 × π | 1,992.7 mm (1.993 m) |
| Revolutions per km | 1,000,000 ÷ 1,992.7 | 502 |
So this tyre turns 502 times per kilometre. That single figure is what the speedometer, odometer, ABS and traction control all work from.
Moving from a factory 225/45R17 to an aftermarket 245/45R18 — a wider tyre on a rim one inch larger.
| Measure | 225/45R17 | 245/45R18 | Change |
|---|---|---|---|
| Sidewall | 101.3 mm | 110.3 mm | +9.0 mm |
| Overall diameter | 634.3 mm | 677.7 mm | +43.4 mm (+6.84%) |
| Circumference | 1.993 m | 2.129 m | +136.3 mm |
| Revolutions per km | 502 | 470 | −32 |
| Speedo reads 100 km/h | 100 km/h true | 106.8 km/h true | +6.8% |
A dual-cab running factory 265/65R17, upgraded to 285/70R17 — the same rim, a wider and taller tyre.
| Measure | 265/65R17 | 285/70R17 | Change |
|---|---|---|---|
| Sidewall | 172.3 mm | 199.5 mm | +27.3 mm |
| Overall diameter | 776.3 mm | 830.8 mm | +54.5 mm (+7.02%) |
| Revolutions per km | 410 | 383 | −27 |
| Speedo reads 100 km/h | 100 km/h true | 107.0 km/h true | +7.0% |
| Ground clearance | — | — | +27.2 mm |
Only half the diameter increase becomes ground clearance — the axle rises by half the change, and the other half goes upward into the guard, which is where clearance problems begin at full suspension compression and full steering lock.
The same 285/70R17 dimensions again, but in an all-terrain or mud-terrain construction rather than a highway tyre. The size arithmetic is identical; the trade-offs are not.
| Gains | Costs | |
|---|---|---|
| All-terrain (A/T) | Better gravel and dirt traction, tougher sidewalls, reasonable on road | More road noise, slightly higher consumption, softer on-road handling |
| Mud-terrain (M/T) | Large void blocks clear mud and clay; strong puncture resistance | Noticeably noisy, longer wet braking distances, faster wear, highest consumption |
The honest question for most owners is how much genuine off-road driving they do. A mud-terrain tyre on a vehicle that spends 95% of its life on bitumen costs fuel, noise and wet grip every single day to deliver traction used a few weekends a year.
Size tells you the dimensions. Type tells you what the tyre is built to do — and the categories that matter in Australia are not the same as those quoted on European or North American sites.
| Type | Marking | Suits | Trade-off |
|---|---|---|---|
| Touring / highway (H/T) | Often unmarked | Most passenger cars and family SUVs | Little off-road ability |
| Performance / summer | Often unmarked | Sports cars, dry and warm conditions | Firmer ride, faster wear, poorer in cold |
| All-season | M+S | Drivers wanting one tyre year-round | Compromised at both temperature extremes |
| All-terrain (A/T) | M+S, sometimes 3PMSF | Utes and 4WDs on gravel, dirt and bitumen | More noise, slightly more fuel |
| Mud-terrain (M/T) | M/T on sidewall | Serious off-road, mud and clay | Loud, longer wet braking, fastest wear |
| Winter | 3PMSF snowflake | Alpine areas only | Wears rapidly in warm conditions |
For the overwhelming majority of Australian drivers the honest choice is between a good touring tyre and, if you genuinely leave the bitumen, an all-terrain. Mud-terrains cost fuel, noise and wet grip every day of the year to deliver traction most owners use a handful of times.
Common Australian sizes, calculated using the same formulas as the tool above.
| Size | Sidewall | Diameter | Circumference | Rev/km | Typically fitted to |
|---|---|---|---|---|---|
| 195/65R15 | 126.8 mm | 634.5 mm | 1.993 m | 502 | Small hatch — Corolla, Yaris, i30 |
| 205/55R16 | 112.8 mm | 631.9 mm | 1.985 m | 504 | Medium hatch and small sedan |
| 215/55R17 | 118.2 mm | 668.3 mm | 2.100 m | 476 | Medium sedan and small SUV |
| 225/45R17 | 101.2 mm | 634.3 mm | 1.993 m | 502 | Sports sedan and warm hatch |
| 235/45R18 | 105.8 mm | 668.7 mm | 2.101 m | 476 | Performance sedan and medium SUV |
| 245/40R19 | 98.0 mm | 678.6 mm | 2.132 m | 469 | Performance car and large SUV |
| 265/65R17 | 172.2 mm | 776.3 mm | 2.439 m | 410 | Dual-cab ute and 4WD wagon |
| 285/70R17 | 199.5 mm | 830.8 mm | 2.610 m | 383 | Off-road 4WD, lifted utes |
Look closely at two rows in that table:
| Size | Overall diameter |
|---|---|
| 195/65R15 | 634.5 mm |
| 225/45R17 | 634.3 mm |
Two rim sizes apart, 30 mm wider — and the rolling diameter differs by 0.2 mm, or 0.03%. That is deliberate. It is called plus sizing: as rim diameter goes up, aspect ratio comes down to hold the overall diameter constant.
This is where tyre sizing stops being a preference and starts being a compliance question.
Australian Design Rule 18/03 requires that a vehicle's speedometer must never indicate a speed lower than the true speed. It may over-read, by up to 10% plus 4 km/h, but under-reading is not permitted. Accuracy is verified at set test speeds during type approval.
Because manufacturers must never breach that floor, they build in a deliberate margin — most new cars read roughly 3–7% above true speed. That margin is the buffer that absorbs tyre wear, pressure variation and small fitment changes.
| Diameter change | Speedo shows 100 km/h — true speed is | Practical view |
|---|---|---|
| +1% | 101.0 km/h | Within normal tolerance |
| +3% | 103.0 km/h | Consuming much of the factory buffer |
| +5% | 105.0 km/h | Likely reading at or near true speed |
| +7% | 107.0 km/h | Very likely under-reading — a real risk |
Because your actual factory over-read is unknown, the practical test is a GPS speed app on a flat road at a steady speed. Compare the dashboard against GPS before and after a tyre change and you will know exactly where you stand.
Speedometer error gets the attention, but the odometer runs off the same count — and its errors accumulate.
Fit tyres 5% larger and every kilometre you travel records as roughly 0.95 km. Over 20,000 km of driving that is about 1,000 kilometres your car never logged. The consequences are quiet but real:
If you have changed tyre size, apply the correction when you track distance — our mileage calculator and gas mileage calculator both assume the odometer is accurate.
| Change | Effect |
|---|---|
| Wider tyre | More rolling resistance and aerodynamic drag; usually more grip, usually more fuel |
| Taller tyre | More rotating mass, lower effective final drive; typically higher consumption |
| Lower profile (shorter sidewall) | Sharper steering response, firmer ride, more vulnerable to pothole damage |
| Higher profile (taller sidewall) | More comfortable, more absorbent over bumps, softer initial steering feel |
| Aggressive tread (A/T, M/T) | Off-road traction gained; road noise, wet braking and consumption all worse |
Larger wheels with shorter sidewalls also mean less rubber between the rim and the road. On Australian secondary roads, an 18- or 19-inch wheel with a 35 or 40 profile is considerably more likely to suffer rim damage from a pothole than a 16-inch wheel with a 60 profile.
An EV places demands on a tyre that a comparable petrol car does not, and fitting a standard tyre to one is a common and expensive mistake.
| EV characteristic | What the tyre needs |
|---|---|
| Battery mass — often 300–500 kg more than an equivalent petrol car | A higher load index, and construction rated for the extra weight |
| Full torque from standstill | Tougher compound and tread design; standard tyres wear noticeably faster |
| Efficiency directly affects driving range | Low rolling resistance — a thirstier tyre costs range, not just money |
| No engine noise to mask road roar | Noise-absorbing foam liners and quieter tread patterns |
| Regenerative braking | Different wear pattern; rotation intervals matter more |
The size arithmetic on this page applies unchanged to electric vehicles. What changes is which tyre within that size you should be fitting.
The characters after the size are the ones most likely to be overlooked and the most safety-critical.
| Load index | Max load per tyre |
|---|---|
| 82 | 475 kg |
| 91 | 615 kg |
| 95 | 690 kg |
| 100 | 800 kg |
| 110 | 1,060 kg |
| 120 | 1,400 kg |
| Speed rating | Maximum speed |
|---|---|
| Q | 160 km/h |
| S | 180 km/h |
| H | 210 km/h |
| V | 240 km/h |
| W | 270 km/h |
| Y | 300 km/h |
The pressures on your placard are specified for the tyre size the vehicle was delivered with. Change size significantly and those figures may no longer be correct, because a different tyre volume and load rating carries the same weight differently.
Six checks, none of which take more than a minute:
| Check | What to look for |
|---|---|
| Tread depth | The legal minimum in Australia is 1.5 mm in the principal grooves. Tyre manufacturers and motoring bodies generally recommend replacing at around 3 mm, because wet braking falls away sharply below that |
| Tread wear indicators | Small raised bars moulded into the grooves. When the tread is flush with them you are at the legal limit — not approaching it |
| Tyre age | The four-digit DOT code gives week and year of manufacture — "2224" means week 22 of 2024. Many manufacturers suggest replacement at around five years regardless of remaining tread, since rubber hardens with age |
| Sidewall condition | Cracking, bulges, cuts or gouges. A bulge means internal structural damage and the tyre should be replaced immediately, whatever the tread looks like |
| Pressure | Monthly, when cold, against the placard on the driver's door frame |
| Uneven wear | Wear on one shoulder usually means alignment; wear in the centre or on both shoulders usually means pressure. Both waste tyres and neither fixes itself |
Two more worth booking at the same time: wheel balancing whenever tyres are fitted, and a wheel alignment if the car pulls, the steering wheel sits off-centre, or the previous set wore unevenly. Both protect the tyres you have just paid for.
Wheel and tyre fitment in Australia is governed partly by the Australian Design Rules and partly by state and territory regulations, and the detail varies depending on where the vehicle is registered.
Commonly applied principles include maintaining speedometer accuracy within ADR 18/03, keeping tyres contained within the vehicle's bodywork, limits on how much wider a rim may be than the widest option the manufacturer offered, and limits on varying aspect ratio between front and rear axles. Suspension lifts beyond a modest height generally require engineering certification.
How do you calculate tyre size?
Multiply the width by the aspect ratio divided by 100 to get sidewall height, then add twice that to the rim diameter converted to millimetres. For 225/45R17: sidewall is 225 × 0.45 = 101.3 mm, and overall diameter is (17 × 25.4) + (2 × 101.3) = 634.3 mm. Multiply diameter by π for circumference.
What does 225/45R17 mean?
225 is the tyre width in millimetres. 45 is the aspect ratio — the sidewall height as a percentage of the width, giving 101.3 mm here. R means radial construction. 17 is the rim diameter in inches. Any characters after that, such as 91V, are the load index and speed rating.
Can I fit bigger tyres to my car?
Often yes, within limits. Keeping the rolling diameter within roughly 1% of standard avoids speedometer and odometer error entirely. Beyond that you need to consider guard clearance at full lock and full suspension compression, wheel offset, load rating, and your state's fitment rules. Check with your manufacturer's fitment guide and a tyre retailer familiar with local requirements before buying.
How much speedometer error is acceptable?
Under ADR 18/03 an Australian speedometer must never indicate below true speed, and may over-read by up to 10% plus 4 km/h. Manufacturers typically build in a 3–7% over-read as a buffer. A tyre change that increases rolling diameter eats into that buffer, so smaller changes are safer. The only reliable check on your own car is comparing the dashboard against a GPS speed reading.
Does tyre size affect fuel economy?
Yes. Wider tyres increase rolling resistance and aerodynamic drag, and taller tyres add rotating mass while lowering the effective final drive ratio. Aggressive off-road tread patterns add more again. Most size increases raise consumption rather than lower it, even though the engine turns fewer revolutions per kilometre.
Does tyre size affect speedometer accuracy?
Directly. Your car counts wheel rotations and multiplies by a circumference figure set at the factory. Larger tyres travel further per rotation, so the speedometer reads lower than your true speed. Smaller tyres do the reverse. A 5% diameter increase means a true 105 km/h when the dashboard shows 100.
What is rolling diameter?
Rolling diameter is the overall diameter of the inflated tyre — rim diameter plus two sidewalls. It is the figure that determines circumference, revolutions per kilometre, and therefore your speedometer and odometer readings. When comparing two tyre sizes, this is the number to match.
What is the tyre aspect ratio?
The aspect ratio is the middle number in a tyre size and expresses sidewall height as a percentage of tread width. A 55 aspect ratio on a 205-wide tyre gives a 112.8 mm sidewall. Lower numbers mean shorter sidewalls, sharper steering response and a firmer ride; higher numbers mean more comfort and more protection from pothole damage.
Can different tyre sizes damage an AWD system?
Yes, and this is one of the more expensive mistakes. All-wheel drive systems expect all four wheels to rotate at matched speeds. Different rolling diameters — from mixed sizes, mismatched wear, or replacing only two tyres — force the centre differential or transfer case to work continuously, which can cause serious damage. Replace all four together on AWD vehicles wherever possible.
Should I change tyre pressure after changing tyre size?
Possibly. Placard pressures are specified for the original tyre size, and a different size with a different volume or load rating may need a different pressure to carry the same weight correctly. Ask your fitter for the recommended pressure for the new size rather than carrying the old figure across, and check monthly when cold.
What is a load rating?
The load index is a coded number indicating the maximum weight a single tyre can carry at its rated pressure — 91 means 615 kg, 110 means 1,060 kg. It matters most on utes, vans and towing vehicles. Never fit a tyre with a load index below the figure on your vehicle's tyre placard.
What is a speed rating?
The speed rating is a letter indicating the maximum speed the tyre is designed to sustain — H is 210 km/h, V is 240 km/h, W is 270 km/h. It reflects construction and heat tolerance rather than being purely about top speed, so a lower-rated tyre is not simply an unused margin. Fitting below the manufacturer's specification may affect roadworthiness and insurance.
The single most useful habit when changing tyres is to compare overall diameter before anything else. Hold it within about 1% of standard and your speedometer, odometer, ABS calibration and gearing all continue working as designed — which leaves you free to choose width, profile and tread pattern on their merits.
Beyond that, weigh the change against how you actually drive. Check what a size change does to your running costs with the fuel cost calculator, re-measure your real consumption afterwards with the gas mileage calculator, and confirm anything unusual with your manufacturer's fitment guide and your state road authority before committing.