Tyre Size Calculator
Read the sidewall, then check the sums before you change size.
Decode a tyre size like 205/55R16 into its overall diameter, circumference and revolutions per kilometre — then drop in a size you're considering to see the rolling-diameter change and how far it would throw your speedo out. The safe target for a plus-size is to stay within about ±3%.
Estimates from the tyre's width, aspect ratio and rim — a planning guide only. Always check your tyre placard and your state's modification rules before changing size.
How overall diameter is worked out
Sidewall first, then the rim
A tyre's overall diameter is the wheel plus a sidewall top and bottom. The sidewall height comes from the first two numbers: sidewall = width × aspect ÷ 100. The rim is quoted in inches, so convert it with × 25.4 to get millimetres. Put them together: overall diameter = (rim × 25.4) + 2 × sidewall. From the diameter, the circumference = π × diameter, and the tyre's revolutions per kilometre = 1,000,000 ÷ circumference (mm) — the figure your speedometer and odometer are geared to.
Worked example
For 205/55R16: sidewall = 205 × 0.55 = 113 mm; rim = 16 × 25.4 = 406 mm; overall diameter = 406 + 2 × 113 ≈ 632 mm. Circumference = π × 632 ≈ 1,985 mm (about 1.99 m), which is roughly 504 revolutions per kilometre. Change any of the three numbers and the diameter moves — a taller sidewall or a bigger rim both make the tyre larger overall.
This is a geometry estimate, not a fitment approval. Real tyres vary a little from their nominal size between brands, and clearance depends on your guards, suspension and offset. Always confirm against the tyre placard and, for anything beyond a like-for-like size, your state's modification rules.
How to read the code on your sidewall
Take 205/55R16 91V as the example. Each part means something:
- 205 — section width in millimetres. How wide the tyre is across the tread. Bigger means a wider contact patch (often more grip) but also more chance of rubbing.
- 55 — aspect ratio (%). The sidewall height as a percentage of the width, so here 55% of 205 mm ≈ 113 mm of sidewall. A lower number is a "lower-profile" tyre with a shorter, stiffer sidewall.
- R — radial construction. Almost every modern car tyre is radial. You may also see the rim as R16 meaning radial on a 16-inch wheel.
- 16 — rim (wheel) diameter in inches. The wheel the tyre is designed to fit.
- 91V — load index and speed rating. 91 is a coded maximum load (here about 615 kg per tyre) and V is a speed rating (up to 240 km/h). Don't fit tyres rated below your placard.
The width and rim are fixed dimensions, but the aspect ratio is a percentage of the width — which is why two tyres on the same 16-inch rim can end up quite different heights. That's the lever plus-sizing pulls: raise the rim, drop the aspect ratio, and keep the overall diameter about the same.
Plus-sizing, the ±3% rule and speedo error
The ±3% rolling-diameter rule
When you change tyre or wheel size, aim to keep the overall (rolling) diameter within about ±3% of standard. Stray outside that and you invite speedo and odometer error, rubbing on guards or suspension at full lock or under compression, and confused ABS and traction control, which read wheel speed. "Plus-sizing" is the trick that keeps you inside the band: go up an inch on the rim while dropping the aspect ratio so the tyre's height barely changes. Worked example: 225/45R17 ≈ 634 mm is only +0.4% versus 205/55R16 (≈632 mm) — a safe plus-size, with a wider, lower-profile look and almost no speedo impact.
How a bigger tyre slows the speedo
Your speedometer is calibrated to the standard tyre's revolutions per kilometre. Fit a bigger tyre and each turn of the wheel covers more ground, so you're actually going faster than the dial shows — a tyre 3% larger reads about 100 km/h when your true speed is roughly 103 km/h. A smaller tyre does the reverse and makes the speedo read high. In Australia the Australian Design Rules require the speedometer never to indicate less than your true speed, so an oversized tyre that makes it under-read is exactly the direction to avoid.
Before you change size (Australia)
Start with the tyre placard — the sticker on the driver's door jamb or inside the fuel flap — and your owner's manual; they list the approved sizes, load and speed ratings and pressures. Keep the new tyre within your state or territory's light-vehicle modification limits (generally based on VSB 14), don't drop below the placard's load index or speed rating, and keep the speedo accurate. If a size sits outside the standard fitments, check with your state road authority or a tyre professional first.
❓ Frequently asked Frequently asked questions
How do I read my tyre size?
A tyre size like 205/55R16 packs three numbers. The first, 205, is the section width in millimetres — how wide the tyre is across the tread. The second, 55, is the aspect ratio: the sidewall height as a percentage of the width, so here the sidewall is 55% of 205 mm ≈ 113 mm. The R means radial construction, and the last number, 16, is the rim (wheel) diameter in inches. You'll often see a load index and speed rating after it, such as 91V. The width and rim are fixed sizes, but the aspect ratio is a percentage, which is why two tyres on the same rim can have very different overall heights.
How do I work out a tyre's overall diameter?
Overall diameter = the rim diameter converted to millimetres (inches × 25.4) plus twice the sidewall height, because there's a sidewall above and below the rim. The sidewall height is width × aspect ÷ 100. For 205/55R16: sidewall = 205 × 0.55 = 113 mm; rim = 16 × 25.4 = 406 mm; diameter = 406 + 2 × 113 ≈ 632 mm. Circumference is π × diameter ≈ 1,985 mm, so the tyre turns about 504 times per kilometre. The calculator does all of this and compares a second size for you.
How much bigger tyre can I fit? (the ±3% rule)
The rule of thumb is to keep the new tyre's overall (rolling) diameter within about ±3% of the original. Beyond that you risk speedo error, rubbing on the guards or suspension at full lock, and upsetting the ABS, traction control and odometer. "Plus-sizing" means going to a larger rim while dropping the aspect ratio to keep the diameter about the same — for example 205/55R16 (≈632 mm) to 225/45R17 (≈634 mm) is only +0.4%, a safe plus-size. Enter your new size and the calculator shows the exact percentage change and flags anything outside ±3%.
Do bigger tyres affect the speedometer?
Yes. The speedometer and odometer are calibrated to the standard tyre's revolutions per kilometre. A larger-diameter tyre covers more ground per turn, so the car is actually going faster than the dial shows — a tyre 3% bigger reads about 100 km/h when you're really doing ~103. A smaller tyre does the opposite and reads high. Australian Design Rules require a speedometer to never indicate less than the true speed, so an oversized tyre that makes the speedo under-read is the direction to avoid — another reason to stay within ±3% and, if anything, slightly under-size rather than over.
Is it legal to change tyre size in Australia?
You can change tyre size in Australia, but the new tyre must stay within the vehicle manufacturer's specifications and the light-vehicle modification limits in your state or territory (generally based on the national code, VSB 14). As a guide, keep the overall rolling diameter within a few percent of standard, don't drop below the load index and speed rating shown on the tyre placard, and make sure the speedometer still reads accurately — that is a legal requirement. Start with the tyre placard on the driver's door jamb or fuel flap and your owner's manual, and if in doubt check with your state road authority or a tyre professional.
Where these figures come from
The maths here is standard tyre geometry, not a country-specific rule. The compliance points are the Australian ones you'll actually be held to.
- Overall diameter — (rim × 25.4) + 2 × (width × aspect ÷ 100), in millimetres; circumference = π × diameter; revs/km = 1,000,000 ÷ circumference.
- Plus-sizing / ±3% rule — the widely used industry rule of thumb for keeping rolling diameter close enough that the speedo, ABS and clearances stay happy.
- Speedometer accuracy — the Australian Design Rules require a speedometer to never read below the true speed (it may read high within a tolerance), so oversized tyres that cause under-reading are the risk.
- Modification limits — your state or territory light-vehicle rules, generally based on the National Code of Practice VSB 14; the tyre placard and owner's manual list your approved sizes, load and speed ratings.
Last checked: July 2026. This is a planning estimate, not modification approval. Nominal tyre sizes vary slightly between brands, and clearance depends on your vehicle. Always confirm against the tyre placard and your state's rules before changing size.
Select the question that matches where you are right now.
The headline number is your tyre's overall diameter. The breakdown shows the sidewall height, circumference and revolutions per kilometre, and — if you entered a new size — the rolling-diameter change as a percentage plus the speedo error at your test speed.
Use the diameter to compare a size you're considering. If the percentage change sits inside ±3% and the load and speed ratings still meet the placard, it's a sensible candidate; confirm clearance and your state's rules before fitting.
It's not a fitment guarantee. It doesn't know your guards, offset or suspension, and real tyres vary a little from their nominal size. Treat it as the geometry check, not the final word.
Your speedo is geared to the standard tyre's revs per kilometre. A bigger diameter covers more ground per turn, so the dial reads slow; a smaller one reads fast. Staying near the original diameter keeps it honest.
Three numbers set the diameter — width, aspect ratio and rim — and only one of them is a percentage.
Because the sidewall is a percentage of the width, dropping the aspect ratio is how a bigger rim keeps the same overall height. That's the whole idea behind plus-sizing.
A wider tyre adds a little sidewall height too (aspect is a % of width), and each extra inch of rim adds 25.4 mm to the diameter before the sidewall is even counted.
Keep the rolling-diameter change inside about ±3% and the speedo, odometer, ABS and clearances stay close to standard. Outside it, expect a noticeable speedo shift and possible rubbing.
A few checks keep a size change safe and legal.
The sticker on the door jamb or fuel flap lists your approved sizes, load index, speed rating and pressures. Match or exceed them — never go below.
Aim for a rolling-diameter change within a few percent. If you want a wider look, plus-size: bigger rim, lower profile, same height.
Anything outside the standard fitments should be checked against your state's VSB 14-based modification limits, and the speedo kept accurate as required by law.
New tyres are one line in the cost of running a car. Model the bigger picture.