acalculator

How do these tire sizes compare?

Enter the numbers from the sidewall of your current tire and of the tire you are thinking of fitting. See how their sizes compare and what your speedometer will really mean.

Your numbers

Units
Diameter difference
1.72%

The new tire’s diameter differs by 1.72%; at a reading of 60 mph you are going 61 mph.

Current diameter
24.97 in
New diameter
25.4 in
Current sidewall
3.99 in
New sidewall
3.7 in
Current circumference
78.45 in
New circumference
79.8 in
Current revolutions per mile
807.6
New revolutions per mile
794
Actual speed on the new tires
61 mph

Diameter difference: 1.72%. The new tire’s diameter differs by 1.72%; at a reading of 60 mph you are going 61 mph.

Diameter difference by new aspect ratio (%)

How to calculate

Computes the overall diameter, sidewall height, circumference and revolutions per mile of two metric tire sizes, the difference between them, and how a speedometer reading changes.

Example with the default inputs (Current width (mm) 225, Current aspect ratio (%) 45, Current rim (in) 17, New width (mm) 235, New aspect ratio (%) 40, New rim (in) 18, Speedometer reading 60 mph): The new tire’s diameter differs by 1.72%; at a reading of 60 mph you are going 61 mph.

Method: sidewall = width × aspect ÷ 100; diameter = rim + 2 × sidewall; circumference = π × diameter; revolutions per mile = 1 mile ÷ circumference; true speed = reading × new diameter ÷ current diameter

  • Sizes are metric tire codes such as 225/45R17: section width in millimetres, aspect ratio in percent, rim diameter in inches.
  • The diameter is the unloaded design diameter. A tire under load, worn, or at a different pressure is a little smaller.
  • The speedometer is assumed to read true on the current tires.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Current width (mm) 225, Current aspect ratio (%) 45, Current rim (in) 17, New width (mm) 235, New aspect ratio (%) 40, New rim (in) 18, Speedometer reading 60 mph gives Current sidewall 3.99 in, Current diameter 24.97 in, New sidewall 3.7 in, New diameter 25.4 in, Diameter difference 1.71843%, Current revolutions per mile 807.61486, Actual speed on the new tires 61 mph.Source: hand calculation in content.mdx: sidewall 225 × 0.45 = 101.25 mm; diameter 431.8 + 202.5 = 634.3 mm (24.97 in); new 457.2 + 188 = 645.2 mm (25.40 in); +1.72%; 60 mph reads 61.03 mph true
  2. Current width (mm) 205, Current aspect ratio (%) 55, Current rim (in) 16, New width (mm) 205, New aspect ratio (%) 55, New rim (in) 16, Speedometer reading 62.14 mph gives Diameter difference 0%, Current diameter 24.88 in, Current revolutions per mile 810.682237, Actual speed on the new tires 62.1 mph.Source: hand calculation in content.mdx: 205 × 0.55 = 112.75 mm; 406.4 + 225.5 = 631.9 mm; 1,609,344 ÷ (π × 631.9) = 810.7 revolutions a mile

How it works

A metric tire size such as 225/45R17 gives the section width W in millimetres, the aspect ratio A in percent, and the rim diameter R in inches.

  • Sidewall height = W × A ÷ 100 (mm).
  • Diameter = R × 25.4 + 2 × sidewall (mm).
  • Circumference = π × diameter.
  • Revolutions per mile = 1,609,344 mm ÷ circumference in mm.
  • Diameter difference = (new diameter ÷ current diameter − 1) × 100%.
  • True speed on the new tires = speedometer reading × new diameter ÷ current diameter. The speedometer is assumed to read true on the current tires.

The chart shows how the diameter difference changes with the new tire's aspect ratio, from 25 to 80, with the other sizes as entered.

These are design sizes for an unloaded tire at its rated pressure. A tire under the car’s weight is flatter at the bottom, and a worn tire is smaller.

Worked examples by hand

225/45R17 against 235/40R18 at a reading of 60 mph.

  • Current: sidewall 225 × 45 ÷ 100 = 101.25 mm. Diameter 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm (24.97 in).
  • New: sidewall 235 × 40 ÷ 100 = 94 mm. Diameter 18 × 25.4 + 2 × 94 = 457.2 + 188 = 645.2 mm (25.40 in).
  • Difference: 645.2 ÷ 634.3 − 1 = +1.72%.
  • Revolutions per mile, current: 1,609,344 ÷ (π × 634.3) = 807.6.
  • True speed: 60 × 645.2 ÷ 634.3 = 61.03 mph.

205/55R16 against itself at 100 km/h. Sidewall 205 × 0.55 = 112.75 mm; diameter 406.4 + 225.5 = 631.9 mm; 1,609,344 ÷ (π × 631.9) = 810.7 revolutions a mile. The difference is 0% and the true speed is 100 km/h.

Other questions people ask

What do the numbers in a tire size mean?

In 225/45R17, 225 is the section width in millimetres, 45 is the aspect ratio (the sidewall height is 45% of the width), R means radial construction, and 17 is the rim diameter in inches.

How do I work out a tire’s diameter?

Sidewall = width × aspect ratio ÷ 100. Diameter = rim + 2 × sidewall, with the rim converted to millimetres (× 25.4). For 225/45R17: 225 × 0.45 = 101.25 mm; 17 × 25.4 + 2 × 101.25 = 634.3 mm, or 24.97 in.

How much can the diameter change?

The calculator shows the change; it does not set a limit. Check your vehicle maker’s and tire maker’s fitment guides: clearance in the wheel well, the load index and the speed rating all matter, not only the diameter.

Why does a bigger tire make my speedometer read low?

The speedometer counts wheel turns. A larger tire covers more ground each turn, so at the same count you travel farther and faster. True speed = reading × new diameter ÷ old diameter.

Does the width change the speedometer?

Only through the sidewall. The sidewall height is the width × the aspect ratio, so a wider tire with the same aspect ratio also has a taller sidewall and a larger diameter.

Why do published diameters differ a little from this one?

This is the design diameter from the size code. Each tire maker publishes a measured diameter for each model, which can differ by a few millimetres, and a loaded tire is shorter than an unloaded one.