Tire size calculator

Reading a tyre size

A marking like 225/45R17 encodes three things:

• 225 — section width in millimetres, sidewall to sidewall.
• 45 — aspect ratio: the sidewall height as a percentage of the width. Here 45% of 225 mm = 101.25 mm.
• R17 — radial construction, on a 17-inch rim.

Overall diameter is the rim diameter plus two sidewalls, since the tyre sits on both sides of the wheel: (17 × 25.4) + (2 × 101.25) = 634 mm.

Note that the aspect ratio is a percentage, not a fixed height. A 245/45 tyre has a taller sidewall than a 225/45 despite the same aspect number, because 45% of a bigger width is more millimetres. This is why widening a tyre without dropping the aspect ratio increases the diameter.

Why the speedometer changes

A speedometer counts wheel revolutions and multiplies by an assumed circumference. Fit a larger tyre and each revolution covers more ground, so you are travelling faster than the dial says. Fit a smaller one and the reverse happens.

The error is proportional. A tyre 3% larger means an indicated 100 km/h is really about 103. Odometer readings drift by the same proportion, which matters for service intervals, warranty mileage and lease agreements.

Most speedometers are already deliberately optimistic from the factory — UNECE and similar regulations permit an indicated speed above the true one but never below, so a small oversize can bring an inaccurate speedometer closer to correct. That is luck rather than a plan.

The ±3% rule

The widely used guideline is to stay within 3% of the original overall diameter. Beyond that:

• ABS and stability control use wheel speed to detect slip, and they are calibrated to the original rolling radius.
• Automatic transmission shift points are speed-referenced.
• Effective gearing changes, altering acceleration and cruising revs.
• Clearance to suspension components and wheel arches can run out at full lock or full compression.

Plus sizing

The usual way to fit a larger rim without changing overall diameter is plus sizing: go up an inch on the rim and drop the aspect ratio by about 10 points, widening slightly. 225/45R17 to 245/40R18 is a textbook example, and this calculator will show it lands close to the original diameter.

The trade-off is real. A shorter sidewall sharpens steering response and reduces roll, but transmits far more road shock into the cabin and leaves the rim much more vulnerable to pothole damage. Larger wheels are also heavier, which increases unsprung mass and usually hurts ride quality and efficiency.

What this does not check

Load index and speed rating are stamped after the size and must be at least equal to the manufacturer's specification — a tyre that fits geometrically can still be unsafe if its load rating is too low. Wheel width, offset and clearance also have to suit, and none of that is visible from the size code alone.

Common questions