Tyre size comparison — diameter, circumference and speedometer error, old vs new.
Read the code off the tyre sidewall: 205 is the width in mm, 55 is the sidewall as a % of the width, and R16 is a 16-inch rim. The speed unit does not matter — the error % is the same in km/h and mph.
Diameter change (speedometer error)
Tyre comparisonHow to use this tool
Read both codes off the tyre sidewalls (or the door sticker for the original size) and type them exactly, for example 205/55R16 and 225/45R17.
Each tyre's diameter, sidewall height, circumference and revolutions per kilometre are shown, so you can see where the difference comes from — usually a taller sidewall or a bigger rim.
Enter a speedometer reading and the calculator shows your true speed. The error % is identical in km/h and mph.
A result within ±3% is generally acceptable; anything larger is flagged so you check fitment, clearance and local rules before buying.
Formula & worked example
Worked example: 205/55R16 has sidewall 205 × 55% = 112.75 mm, rim 16 × 25.4 = 406.4 mm, diameter 406.4 + 2 × 112.75 = 631.9 mm and circumference π × 631.9 = 1985.2 mm. Changing to 225/45R17 gives sidewall 101.25 mm, rim 431.8 mm, diameter 634.3 mm — a change of +2.4 mm, or +0.38%. At an indicated 100, you are really doing 100.38, so the speedometer error is +0.38%.
Limitations — read this before you rely on it
This calculator uses the nominal tyre dimensions from the code; real tyres vary slightly by brand, wear and inflation pressure, so treat the last millimetre as approximate. It does not check physical fitment — brake clearance, arch clearance, suspension travel, load index and speed rating must all be confirmed for your vehicle. The ±3% figure is a widely used guideline, not a law; local regulations vary.
Frequently asked questions
A tire size calculator compares two tyre sizes, old versus new. Enter both codes (for example 205/55R16 and 225/45R17) and it returns each tyre's diameter, circumference, sidewall height and revolutions per kilometre, plus the diameter difference and the speedometer error at any speed.
The code is width / aspect ratio R rim diameter. In 205/55R16: 205 mm wide, sidewall height 55% of the width (112.75 mm), radial construction (R), on a 16-inch rim. Diameter = rim + two sidewalls = 406.4 + 225.5 = 631.9 mm.
Your speedometer and odometer are calibrated to the original tyre's circumference. A larger new tyre covers more ground per revolution, so at the same wheel speed you travel faster than the speedometer says. The error % equals the diameter change %: +0.38% here means 100 indicated is really 100.38.
As a rule of thumb, stay within ±3% of the original diameter. Beyond that, speedometer and odometer error, gearing, clearance and in some places legality all become real concerns. Always confirm the new size clears the brakes and arches and is approved for your vehicle.
Speedometer error is a percentage, so the unit cancels out: +0.38% means 100.38 whether you read 100 km/h or 100 mph. Enter the speed in whichever unit your speedometer uses.