Tire (Tyre) Size Calculator
Decode any tire size into its width, sidewall height, overall diameter and circumference, then compare it with another size to see the speedometer and odometer error.
How a tire size is read
Example: 205/55R16 91V
Popular tire sizes and their dimensions
| Size | Sidewall (mm) | Diameter (mm) | Diameter (in) | Circumference (mm) | Revs / km |
|---|---|---|---|---|---|
| 175/65R14 | 113.8 | 583.1 | 23 | 1832 | 546 |
| 185/65R15 | 120.3 | 621.5 | 24.5 | 1952 | 512 |
| 195/65R15 | 126.8 | 634.5 | 25 | 1993 | 502 |
| 205/55R16 | 112.8 | 631.9 | 24.9 | 1985 | 504 |
| 215/60R16 | 129 | 664.4 | 26.2 | 2087 | 479 |
| 225/45R17 | 101.3 | 634.3 | 25 | 1993 | 502 |
| 225/65R17 | 146.3 | 724.3 | 28.5 | 2275 | 439 |
| 235/55R18 | 129.3 | 715.7 | 28.2 | 2248 | 445 |
| 245/45R18 | 110.3 | 677.7 | 26.7 | 2129 | 470 |
| 255/55R19 | 140.3 | 763.1 | 30 | 2397 | 417 |
| 265/65R17 | 172.3 | 776.3 | 30.6 | 2439 | 410 |
| 275/55R20 | 151.3 | 810.5 | 31.9 | 2546 | 393 |
| 265/70R16 | 185.5 | 777.4 | 30.6 | 2442 | 409 |
| 285/75R16 | 213.8 | 833.9 | 32.8 | 2620 | 382 |
Speed ratings
| Letter | Q | R | S | T | U | H | V | W | Y |
|---|---|---|---|---|---|---|---|---|---|
| Max speed (km/h) | 160 | 170 | 180 | 190 | 200 | 210 | 240 | 270 | 300 |
How the calculation works
Sidewall height = width × aspect ratio ÷ 100. Overall diameter = rim diameter (inches × 25.4) + 2 × sidewall height. Circumference = π × diameter, and revolutions per kilometre = 1,000,000 ÷ circumference in millimetres. Because the speedometer and odometer count wheel rotations, the speed error between two sizes is the ratio of their circumferences.
Inch sizes such as 33x12.50R15 give the overall diameter and section width directly, so the calculator uses those values and works out the aspect ratio for comparison.
Two worked examples
The original tire is 631.9 mm tall. The new one is 634.3 mm. That's a difference of 2.4 mm, or 0.4%. The wheel is an inch bigger and the tire is wider, but the overall height barely changes, so your speedometer will read 100 km/h when you're really doing 100.4 km/h. This is exactly how plus-sizing is meant to work.
The original is 802.8 mm tall. The 33-inch tire is 838.2 mm, which is 4.4% bigger. When the speedometer says 100 km/h you're really doing about 104.4 km/h. The odometer under-reads, so every 100 km you drive shows up as about 96 km on the clock. The engine also turns about 4% slower at the same road speed, so the vehicle feels lazier in its gears. That's a very different result from the first example, and it's why this calculator compares two tires, not just one.
A plus-size ladder from 205/55R16
These are common sizes that keep the overall height close to a 205/55R16. Availability varies by brand and market, so check what your tire shop can get.
| Size | Diameter (mm) | Difference | Real speed when speedo reads 100 |
|---|---|---|---|
| 205/55R16 | 631.9 | 0.0% | 100.0 km/h |
| 205/50R17 | 636.8 | +0.8% | 100.8 km/h |
| 215/45R17 | 625.3 | −1.0% | 99.0 km/h |
| 225/45R17 | 634.3 | +0.4% | 100.4 km/h |
| 215/40R18 | 629.2 | −0.4% | 99.6 km/h |
| 225/40R18 | 637.2 | +0.8% | 100.8 km/h |
| 235/35R19 | 647.1 | +2.4% | 102.4 km/h |
Why overall diameter is the number that matters
Everything on your car that counts wheel turns assumes a certain tire height: the speedometer, the odometer, the gearbox, the traction control and the ABS. Change the height and you change what they all see. Width changes how the tire feels and grips. Height changes how the whole car behaves. That's why the rule of thumb is to keep the new tire's diameter within about 3% of the old one.
In many countries the rules say a speedometer may read a little high but must never read low. A bigger tire pushes it the other way, making it read low, so check the rules where you live before you change.
Things to check besides diameter
- Rim width. Every tire width has a range of rims it's approved for. A 225 tire on a rim that's too narrow or too wide changes the shape of the tire and how it handles. Check the tire maker's range.
- Load index and speed rating. The new tire should be at least as high as the one on the car's door-pillar sticker or in the manual.
- Clearance. Check the wheel arch and suspension at full steering lock and when the suspension is compressed. Bigger rims also need clearance for the brake callipers.
- All-wheel drive. Many AWD systems need all four tires to have almost the same rolling circumference. Mismatched tires can strain the drivetrain, so check the manual and replace all four.
- The wheel itself. A new wheel size also changes where the wheel sits. Use the wheel offset calculator for that.
A word on low-profile tires and potholes
A lower aspect ratio means a shorter sidewall. That sharpens the steering and looks great, but there's less rubber and air between the road and the rim. On a road with potholes, a 35 or 40 profile is much more likely to bend a rim or damage the tire than a 55 or 60. The calculator shows you the sidewall height in millimetres, so you can see just how much cushion you're giving up.
A tire that's a few percent too big or small changes your speed reading, your fuel use and how your safety systems behave, and you don't notice it until something goes wrong. Five minutes comparing two tires before you buy is cheap insurance.
Related tools
Frequently asked questions
What do the numbers on a tire mean?
In a size such as 205/55R16, 205 is the section width in millimetres, 55 is the aspect ratio (the sidewall height as a percentage of the width), R means radial construction, and 16 is the rim diameter in inches. Sizes are often followed by a load index and a speed rating, for example 91V.
How do I calculate tire diameter?
Multiply the width by the aspect ratio and divide by 100 to get the sidewall height in millimetres. Double it, because there is a sidewall above and below the rim, and add the rim diameter converted to millimetres (rim inches × 25.4). For 205/55R16: sidewall = 205 × 0.55 = 112.75 mm, so the overall diameter = 2 × 112.75 + 16 × 25.4 = 631.9 mm (24.9 inches).
How much does the speedometer change with a different tire size?
The speedometer counts wheel rotations, so a larger overall diameter means you travel further per rotation and the real speed is higher than the display. The error is the ratio of the two circumferences. A tire that is 3% larger in diameter makes a speedometer reading of 100 km/h correspond to about 103 km/h of real speed, and the odometer under-reads distance by the same 3%.
How much larger a tire can I fit?
A common guideline is to keep the new overall diameter within about 3% of the original, which keeps the speedometer, gearing, ground clearance and ABS or traction control close to what the car was designed for. Check wheel-arch clearance and the vehicle manual or a tire fitter before changing size.
What is the difference between inch sizes like 33x12.50R15 and metric sizes?
Inch (flotation) sizes give the overall diameter, the section width and the rim diameter in inches, while metric sizes give the width in millimetres, the aspect ratio and the rim in inches. This calculator reads both. For an inch size it also shows the nearest metric dimensions, but those may not exist as real tires, so use them as a guide and compare the diameter and width.