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Tire Size Calculator

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A marking such as 225/45R17 mixes two measurement systems in one string: the width is millimetres, the aspect ratio is a percentage of that width, and the rim is inches. Because the sidewall height depends on the width, changing either number changes the overall diameter — and the speedometer, which counts wheel revolutions, is wrong by exactly that proportion. Enter two sizes and this compares them on every measurement that matters.

Width in mm / aspect ratio / rim in inches

The size you are considering

Common sizes:

How to use the tire size calculator

  1. 1Enter your current tyre size as it is written on the sidewall — 225/45R17, or 225/45/17.
  2. 2Enter the size you are considering.
  3. 3Press Compare.
  4. 4Read the diameter difference as a percentage: most fitment guidance treats 3% as the limit.
  5. 5Check the speedometer rows, which show what the instrument will read and what you will actually be doing.

Examples

A typical plus-size

Input
225/45R17 → 225/40R18
Result
+0.46% diameter — well inside tolerance

The larger rim is offset by the shorter sidewall, which is exactly what plus-sizing sets out to do. Going to 235/40R18 instead would be +1.72%, because the wider tyre adds sidewall back.

A wider tyre, same rim

Input
205/55R16 → 225/55R16
Result
+3.48% — outside the usual 3% limit

The aspect ratio is a percentage of the width, so a wider tyre at the same aspect ratio is also a taller one.

Where the speedometer notices

Input
195/65R15 → 205/65R15
Result
Speedometer reads 98.0 at a true 100

A larger tyre travels further per revolution, so the instrument under-reads and you are going faster than it says.

About the tire size calculator

Plus-sizing, and what it is trading

The idea behind plus-sizing is to fit a larger wheel while keeping the overall diameter unchanged, by reducing the aspect ratio to compensate. Going from a 17-inch rim to an 18 adds about 25 mm to the diameter, so the sidewall has to lose about 12.5 mm per side to break even — which is roughly what dropping the aspect ratio from 45 to 40 achieves at a 225 width.

The reason to do it is handling. A shorter sidewall flexes less under cornering load, so steering response sharpens and the car feels more precise. It also exposes more wheel, which is the aesthetic argument and, in practice, often the real one.

The costs are real and worth stating. Less sidewall means less compliance over bumps, so the ride is firmer and the wheel is more vulnerable to pothole damage. The larger wheel is usually heavier, and that weight is unsprung and rotating, which hurts both ride and acceleration slightly. And a low-profile tyre in a large diameter is generally more expensive to replace.

Reading the numbers as measurements rather than as a label

Two of the three numbers are dimensions and one is a ratio, which is the source of most confusion about tyre sizes. The width and the rim are directly comparable between sizes: 225 mm is always wider than 205, and 18 inches is always a larger rim than 17. The aspect ratio is not — a 40 is not shorter than a 45 in any absolute sense, because it depends entirely on what it is 40% of.

This is also why the circumference matters more than the diameter for anything to do with distance. The odometer, the ABS wheel-speed comparison and the traction control all work in revolutions, and revolutions per kilometre is set by the circumference. A 1% change in diameter is a 1% change in circumference and a 1% error in every distance-derived figure the car reports.

One thing the size code deliberately does not encode is the loaded radius — how far the axle actually sits above the ground with the car's weight on the tyre. That is smaller than the free radius because the tyre deflects, it varies with pressure and load, and it is what genuinely determines ground clearance and gearing. The figures here are the nominal geometry, which is what the size code describes and what every comparison chart uses.

Frequently asked questions

What do the three numbers actually mean?
The first is the section width in millimetres, measured at the widest point of the sidewall. The second is the aspect ratio — the sidewall height as a percentage of that width, not as a measurement in its own right. The third is the rim diameter in inches. So a 225/45R17 has a 101.25 mm sidewall, because 45% of 225 is 101.25, and an overall diameter of the 17-inch rim plus two of those sidewalls.
Why does a wider tyre also become taller?
Because the aspect ratio is a proportion rather than a fixed height. Going from 205 to 225 mm wide at the same 55 aspect ratio takes the sidewall from 112.75 mm to 123.75 mm — eleven millimetres taller per side, so twenty-two on the diameter. This catches people out regularly: fitting a wider tyre without dropping the aspect ratio changes the rolling diameter and the speedometer with it.
How much difference in diameter is acceptable?
Three percent is the figure most fitment guidance uses, and it is a rule of thumb rather than a regulation. Within it, the speedometer error stays small, the ABS and traction control calculations remain valid, and clearance is unlikely to change enough to matter. Beyond it, expect a noticeable instrument error and check physically that the tyre clears the arch and suspension at full lock and full compression — a calculation cannot tell you that.
Why does the speedometer read wrong at all?
Because it does not measure speed directly. It counts wheel or driveshaft revolutions and multiplies by the rolling circumference the manufacturer assumed. Fit a larger tyre and each revolution covers more ground than the instrument thinks, so it under-reads — and the odometer under-counts by the same proportion, which matters for service intervals and resale mileage. A smaller tyre does the reverse. Most speedometers are also deliberately calibrated to read slightly high from the factory, so a small increase in diameter can bring the reading closer to the truth rather than further from it.
Does this tell me whether a size will fit my car?
No, and nothing derived from the size code can. Fitment depends on the wheel width and offset, the clearance to the strut, arch and brake caliper through the full range of suspension travel and steering lock, and on the load index and speed rating being adequate for the vehicle. Those come from the manufacturer's approved list. What this gives you is the geometry, which is the part that decides whether a candidate size is worth investigating at all.

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