Tire Size Calculator (compare tyre sizes & speedometer error)

Understanding the tyre size calculator — concept and principle

This calculator compares two tyre sizes, such as your current tyres and a planned upgrade. It shows each tyre's overall diameter and sidewall height, how many times it turns per kilometre, and how much the speedometer will be off with the new size. Use it before fitting larger wheels or different tyres.

Overall diameter = rim + 2 sidewalls; here it changes by only 0.38 %.

How it is calculated

For a size such as 205/55R16:

Sidewall height = width × aspect ratio ÷ 100 = 205 × 55 ÷ 100 mm

Overall diameter = rim diameter × 25.4 + 2 × sidewall height

Speedometer difference = (new diameter ÷ old diameter − 1) × 100 %

True speed = speedometer reading × new diameter ÷ old diameter

Revolutions per km = 1,000,000 ÷ (π × diameter in mm)

205 is the section width in mm, 55 the sidewall height as a percentage of the width, and 16 the rim diameter in inches.

The principle

A car's speedometer counts wheel turns and assumes each turn covers the circumference of the original tyre. A taller tyre covers more ground per turn, so the car moves faster than the dial shows; a shorter tyre does the opposite. Circumference is π × diameter, so the error equals the ratio of the two diameters. When people fit bigger rims, they usually choose a lower aspect ratio so the overall diameter stays nearly the same; this is called plus sizing.

Worked example

Default inputs: current 205/55R16, new 225/45R17, reading 100.

Current sidewall = 205 × 0.55 = 112.8 mm; diameter = 16 × 25.4 + 2 × 112.75 = 406.4 + 225.5 = 631.9 mm.
New sidewall = 225 × 0.45 = 101.3 mm; diameter = 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm.
Difference = 634.3 ÷ 631.9 − 1 = 0.38 %, which is within ±3%.
True speed at a reading of 100 = 100 × 1.0038 = 100.4.
Revolutions per km: 504 for the current tyre and 502 for the new one.

How to read the result

A positive difference means the new tyre is taller: at an indicated 100 you are really going a little faster. A negative difference means you go slower than shown. Many tyre fitters aim to keep the change within about ±3 %. Diameter is only one check: the new tyre must also meet the car's load index and speed rating and fit without rubbing at full steering lock and suspension travel.

Common mistakes

Reading the aspect ratio as millimetres instead of a percentage of the width.
Forgetting that the rim size is in inches while the width is in millimetres.
Assuming a tyre that fits the rim will also clear the wheel arch.
Ignoring the load index and speed rating printed on the sidewall.

Keep in mind: Diameters here are nominal; real tyres vary slightly by brand and wear. Approved sizes are listed by the vehicle maker, and a qualified tyre fitter should confirm any change.

Frequently asked questions

What does 205/55R16 mean?

205 mm wide, sidewall 55 % of the width, radial construction, for a 16-inch rim.

Will bigger tyres make my speedometer wrong?

Taller tyres make it read lower than your true speed; the calculator shows by how much.

Can I fit 225/45R17 instead of 205/55R16?

The diameters differ by only 0.38 %, a common plus-one swap, but check the car maker's approved sizes and wheel offset.

Sources: ISO 4000-1, Passenger car tyres and rims — Part 1: Tyres (metric series) · ETRTO Standards Manual (tyre size designation)

Checked 9 Nov 2026. Metric tyre size designation of ISO 4000-1 and the ETRTO Standards Manual (nominal section width in mm / aspect ratio in % / rim diameter code in inches); 1 inch = 25.4 mm.

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