How tire size is calculated
A tire like 225/45 R17 means a 225 mm tread width, a sidewall that's 45% of that width (the aspect ratio), mounted on a 17-inch wheel. The overall diameter is the wheel diameter plus two sidewalls:
diameter = (wheel × 25.4) + 2 × (width × aspect ÷ 100)
Change the diameter and you change your true speed at any given speedometer reading, your effective gearing, and how much rubber sits inside the arch. That's why fitment guides push the ±3% rule — stay inside it and the car behaves the way the factory intended.
Tire size comparison chart
How the common sizes stack up, generated by running the calculator above so a row here cannot disagree with the tool.
Common sizes against a 225/45 R17
Anything inside ±3% keeps your speedometer honest and your arches quiet. Change the original tyre above and the tool recalculates for your car — this chart is the common ground.
| Size | Overall Ø | vs 225/45 R17 | Circumference | Revs / mile |
|---|---|---|---|---|
| 195/65 R15 | 634.5 mm 24.98 in | 0.0% | 1,993 mm 78.5 in | 807 |
| 205/55 R16 | 631.9 mm 24.88 in | -0.4% | 1,985 mm 78.1 in | 811 |
| 205/50 R17 | 636.8 mm 25.07 in | 0.4% | 2,001 mm 78.8 in | 804 |
| 215/45 R17 | 625.3 mm 24.62 in | -1.4% | 1,964 mm 77.3 in | 819 |
| 225/45 R17 | 634.3 mm 24.97 in | 0.0% | 1,993 mm 78.5 in | 808 |
| 225/40 R18 | 637.2 mm 25.09 in | 0.5% | 2,002 mm 78.8 in | 804 |
| 235/40 R18 | 645.2 mm 25.40 in | 1.7% | 2,027 mm 79.8 in | 794 |
| 235/35 R19 | 647.1 mm 25.48 in | 2.0% | 2,033 mm 80.0 in | 792 |
| 245/40 R18 | 653.2 mm 25.72 in | 3.0% | 2,052 mm 80.8 in | 784 |
| 245/35 R19 | 654.1 mm 25.75 in | 3.1% | 2,055 mm 80.9 in | 783 |
| 255/35 R19 | 661.1 mm 26.03 in | 4.2% | 2,077 mm 81.8 in | 775 |
| 255/40 R18 | 661.2 mm 26.03 in | 4.2% | 2,077 mm 81.8 in | 775 |
| 265/35 R18 | 642.7 mm 25.30 in | 1.3% | 2,019 mm 79.5 in | 797 |
| 275/35 R19 | 675.1 mm 26.58 in | 6.4% | 2,121 mm 83.5 in | 759 |
| 285/30 R20 | 679.0 mm 26.73 in | 7.0% | 2,133 mm 84.0 in | 754 |
Plus-sizing without changing the rolling diameter
Plus-sizing is going up a wheel diameter while dropping the sidewall to keep the overall diameter almost the same. Done properly the speedometer stays honest and the gearing does not change — you are only trading sidewall for rim.
The chart above is that trade written out. Going from a 225/45 R17 to a 245/40 R18 takes you to 653.2 mm overall — +3.0%, right on the edge of the tolerance. Drop to a 235/40 R18 instead and you land far closer. That is the whole skill: each inch of rim adds 25.4 mm of diameter, and you need to take roughly 12.7 mm out of each sidewall to pay for it.
What you get is sharper turn-in and a bigger brake package clearance, because there is less sidewall to flex. What you pay is ride quality, kerb damage risk and usually weight — and rotating unsprung weight costs acceleration more than the same mass in the boot.
Why ±3% is the number everyone quotes
It is a tolerance stack, not a law of physics. Inside about ±3% the speedometer error stays within what most legislation permits, the ABS and stability systems keep working from wheel-speed assumptions that are still roughly true, and the tyre stays inside the arch through full compression and full lock.
Push past it and the failures arrive in that order: speedometer first, then rubbing at full lock or over bumps, then — on a car that reads road speed from the wheels — a gearbox that shifts at slightly the wrong moment and a traction control system working from a wrong baseline. The tool flags the percentage directly so you do not have to guess where you are.
Speedometer error, and which way it lies
Your speedometer counts wheel revolutions and multiplies by the factory tyre's circumference. Change the tyre and that assumption is wrong in a specific direction:
- A taller tyre covers more ground per revolution, so you are travelling faster than the dial says. The speedo under-reads — this is the direction that collects speeding tickets.
- A shorter tyre does the reverse and the speedo over-reads, which is harmless legally and annoying on a long drive.
Most cars leave the factory reading 2–5% optimistic on purpose, because regulations in many markets allow a speedometer to over-read but never to under-read. That factory margin is why a modest taller tyre often brings a speedometer closer to true rather than further from it — and why the honest way to check is a GPS, not the dashboard.
What the width number does not tell you
Section width is the widest point of the inflated tyre, not the tread. It is also quoted for a specific measuring rim, so the same 245 fitted to an 8J and a 9.5J will measure differently and sit differently in the arch. Two 245s from different manufacturers can differ by several millimetres before you fit them to anything.
Which is why fitment is a two-part question: this tool tells you what the size change does to diameter and gearing, and the wheel offset calculator tells you where the assembly ends up relative to your strut and your arch lip. You need both answers before you buy.
FAQ
Why does a bigger tire make my speedometer wrong?
Your speedometer counts wheel revolutions and assumes the factory tire's circumference. A taller tire travels further per revolution, so you're actually going faster than the dial shows — the speedo under-reads. A shorter tire does the opposite.
How much tire size change is safe?
As a rule of thumb, keep the overall diameter within ±3% of the original. Beyond that you'll notice speedometer error, altered gearing, and a higher chance of rubbing on suspension or bodywork at full lock or compression.
Is a "plus size" the same diameter?
Done properly, yes. Plus-sizing goes up in wheel diameter while dropping the aspect ratio to keep the overall diameter nearly identical — bigger wheel, shorter sidewall, same rolling circumference.