Reading 225/45R17
The code looks like one measurement and is three, in three different units, in a row. That is most of why it resists intuition.
The parts of a metric tyre designation, and the unit each is in| Part | Means | Unit |
|---|
| 225 | Section width, at the widest point | Millimetres |
|---|
| 45 | Aspect ratio — sidewall height as a share of the width | Percent |
|---|
| R | Radial construction | — |
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| 17 | Rim diameter | Inches |
|---|
So a 225/45R17 has a sidewall of 45% of 225 mm, which is 101.25 mm, and an overall diameter of the 17-inch rim (431.8 mm) plus two sidewalls — 634.3 mm, or just under 25 inches.
Two things follow that catch people out. A tyre’s stated width is the section width at its widest bulge, not the tread width, so the contact patch is narrower than the number. And the rim figure is a diameter in inches sitting inside a code otherwise in millimetres, which is why you cannot compare two sizes by eye without doing the arithmetic.
The middle number is a percentage
This is the most misread part of the code. “45 profile” sounds like a fixed sidewall height, and it is not — it is a proportion, so the same profile number on a wider tyre gives a taller sidewall.
Three tyres that all say “45”, on the same 17-inch rim| Size | Sidewall | Overall diameter |
|---|
| 195/45R17 | 87.8 mm | 607.3 mm |
|---|
| 225/45R17 | 101.3 mm | 634.3 mm |
|---|
| 275/45R17 | 123.8 mm | 679.3 mm |
|---|
Seventy-two millimetres of diameter between the first and the last, all of them “45 profile” on the same rim. Which means widening a tyre without changing the profile makes it taller too— a common mistake when someone wants a wider footprint and assumes the rest stays put. Going from 225 to 245 at the same profile and rim adds 18 mm of diameter, which is 2.8% and very nearly out of tolerance on its own.
To go wider and keep the diameter you have to drop the profile at the same time, which is exactly what the alternatives list in the calculator is doing.
Why your speedometer starts lying
A speedometer does not measure speed. It counts wheel revolutions and multiplies by the rolling circumference it was calibrated for at the factory. Change the tyre and the multiplier is wrong, silently and at every speed.
Fit 225/50R17 in place of 225/45R17 and the diameter goes from 634.3 to 656.8 mm — 3.55% bigger. Each revolution now covers 3.55% more ground than the instrument believes, so at an indicated 100 km/h you are actually doing 103.5. On a 70 mph limit that is a genuine margin between legal and not.
The odometer is wrong too, and — this is the part that surprises people — by a different number. The speed error is +3.55%; the distance error is −3.43%, because the two are reciprocals rather than the same figure. Every journey logs 3.43% shorter than it really was, which quietly moves service intervals, warranty mileage and resale value.
One complication worth knowing, because it cuts the other way. Speedometers are regulated to never under-read, so manufacturers build in a margin and the instrument in front of you already reads a few percent high. A modest increase in rolling diameter therefore often makes the reading more accurate rather than less — but you cannot know by how much without checking against a GPS, and “it was probably reading high anyway” is not a calculation.
And the effect is not confined to the dashboard. ABS, traction control, stability control and cruise control all take wheel speed as an input. A change big enough to matter to the speedometer is a change to what those systems think the car is doing.
Plus sizing, and the ±3% rule
“Plus sizing” is fitting a larger rim while shortening the sidewall by the same amount, so the overall diameter stays put. Done properly the speedometer never notices.
The classic ladder is worth seeing written down, because the numbers look unrelated and the tyres are almost the same size:
Three sizes with a rim spread of two inches and near-identical rolling diameter| Size | Rim | Sidewall | Diameter |
|---|
| 195/65R15 | 15″ | 126.8 mm | 634.5 mm |
|---|
| 205/55R16 | 16″ | 112.8 mm | 631.9 mm |
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| 225/45R17 | 17″ | 101.3 mm | 634.3 mm |
|---|
Two and a half millimetres separate the largest from the smallest — four tenths of a percent. These three are genuinely interchangeable on rolling diameter, which is why they turn up together on so many cars as trim-level options.
The conventional limit for a substitution is ±3% of overall diameter. That figure is a rule of thumb rather than a legal threshold, and it exists because it is roughly where speedometer error, ABS calibration and arch clearance all start to become real rather than academic. Staying inside 1% is better if you have the choice, and the calculator sorts alternatives closest-first for that reason.
What you gain and lose is predictable. A bigger rim with a shorter sidewall sharpens steering response and looks the way most people want it to; it also rides harder, transmits more road noise, makes the wheel far easier to kerb, and costs more to replace. There is no version of this trade that is free.
What the geometry cannot tell you
Everything above is arithmetic on three numbers. Fitting a tyre involves several things that no amount of arithmetic can settle, and it is worth being explicit about where the calculator stops.
Rim width. A wheel accepts a range of tyre widths, typically spanning about 30 mm. A tyre outside that range sits wrongly on the rim whatever its diameter works out to — which is why the alternatives list here filters for plausible width as well as matching diameter, and why even that filter is a plausibility check rather than a guarantee.
Load index and speed rating. The letters and numbers after the size are not decoration. A tyre with a lower load index than the vehicle requires is unsafe and, in most places, illegal — and it is entirely possible to find a perfect diameter match with the wrong rating.
Clearance. A wider or taller tyre has to clear the arch at full lock and full suspension compression, not just standing still in the driveway, and it has to clear the brake caliper on the inside.
The manufacturer’s list. The placard in the door frame or fuel flap names the sizes the vehicle was approved with. In many countries departing from it affects roadworthiness and insurance regardless of whether the geometry works.
So use this page to understand what a change would do, and a tyre professional and that placard to decide whether to make it.
Sources and methodology
The arithmetic is geometry and needs no authority — a sidewall is a percentage of a width, and a diameter is a rim plus two sidewalls. What the sources settle is the designation itself, which is a standard rather than a convention anyone could have guessed, and the speedometer regulation, which explains why the instrument in front of you already reads high before you change anything.