Tire Size Calculator

Reading the tire size code

A tyre size such as 205/55 R16 encodes three measurements. The first number (205) is the tyre width in millimetres. The second (55) is the aspect ratio - the sidewall height as a percentage of the tyre width. The third (16) is the rim diameter in inches.

Section height in millimetres equals width multiplied by the aspect ratio as a decimal. Overall diameter equals the rim diameter converted to millimetres plus two section heights - one each side of the rim.

The calculator accepts the size as separate width, aspect-ratio, and rim inputs, and it also lets you paste a metric code such as 225/45R17. That matters for real users because the code printed on the sidewall is usually the quickest reliable input, while the separate fields make it easier to test plus sizing and winter wheel alternatives.

Speedometer error with a different tire size

Your speedometer is calibrated for the original tyre circumference. Fitting a tyre with a larger diameter means the vehicle travels further per wheel revolution than the speedometer expects, so the speedo under-reads actual speed.

The speedometer error percentage is the difference in circumference divided by the new circumference. A change beyond the common 3% comparison band is significant enough to deserve careful checking because it can affect odometer readings, gearing feel, clearance, and systems that assume the original rolling diameter.

The result panel gives the calculation both ways: what the speedometer would show at an actual 100 km/h, and what your real road speed would be when the dashboard shows 100 km/h or 60 mph. That direct speedometer correction is easier to act on than a percentage alone.

What revs per mile and ground clearance add

Many tire size comparison calculators stop at diameter and sidewall height. Revolutions per mile adds another useful view because it tells you how many wheel rotations the vehicle sees over a fixed distance. A larger tyre has fewer revolutions per mile; a smaller tyre has more. That same relationship is why speedometer and odometer readings shift.

Ground-clearance change is half of the diameter difference. If the replacement tyre is 20 mm taller overall, the axle and body sit about 10 mm higher before load, pressure, tread design, and suspension geometry are considered. That is useful for off-road and winter tyre planning, but it is not a full clearance guarantee.

Worked example: 205/55R16 to 225/45R17

Suppose the original tyre is 205/55R16 and the replacement is 225/45R17. The original sidewall is 205 x 0.55 = 112.75 mm. The overall diameter is 16 x 25.4 + 2 x 112.75 = 631.9 mm.

The 225/45R17 replacement has a sidewall of 101.25 mm and an overall diameter of 634.3 mm. That is only about 2.4 mm taller, or roughly 0.38% larger in diameter, which keeps the rolling circumference very close. The replacement is wider and has a shorter sidewall, so the calculator flags both the physical width change and the small ride-height change rather than pretending diameter is the only decision.

In this example, speedometer error is small because the circumference is close. The bigger practical questions are whether the 225 mm section width suits the wheel width, whether the offset keeps the tyre clear of arches and suspension, and whether the load index and speed rating match the vehicle requirement.

Using the 3% diameter band wisely

A tire size calculator is a planning screen, not a fitment approval. The common plus-sizing rule of thumb is to keep the replacement overall diameter close to the original, often within about 3%, so speedometer and clearance effects remain manageable. That threshold is useful because it makes a first-pass comparison fast, but it is not a universal legal or manufacturer approval.

Final fitment still depends on the exact tyre model, wheel width, wheel offset, suspension travel, brake clearance, load index, speed rating, and vehicle placard. The calculator therefore labels close diameter matches as planning results and tells you when the size falls outside the common comparison band, rather than giving a false yes/no guarantee.

Common mistakes when comparing tire sizes

The most common mistake is comparing only rim diameter. A 17-inch wheel can carry many different outside diameters depending on width and aspect ratio, while a smaller or larger rim can still produce almost the same total rolling diameter if the sidewall changes in the opposite direction.

Another mistake is ignoring tyre width. A replacement that matches diameter can still rub if it is much wider, especially with a different wheel offset. Likewise, a narrower winter tyre may be intentional even when the overall diameter stays close, because winter packages often trade width for clearance and snow performance.

Finally, remember that nominal tire size is not a perfect measured dimension. Different tyre models in the same printed size can vary slightly, especially across tread pattern, construction, load range, and measuring rim. Use the calculator for geometry, then check the tyre manufacturer's specification sheet for the exact product.