Motorcycle Tyre Size Calculator

How to Read a Motorcycle Tyre Size Code

A tyre code like 160/60/17 breaks down simply. The first number (160) is the tyre width in millimetres. The second (60) is the aspect ratio — the sidewall height as a percentage of the width. The third (17) is the rim diameter in inches.

Does Tyre Size Affect Speedo Reading?

Yes. A larger overall diameter means the wheel covers more ground per revolution, so your speedo reads lower than your actual speed. A smaller diameter makes the speedo over-read. This tool shows the exact percentage difference so you know what to expect after a tyre swap.

Working Out Overall Diameter

Rim diameter is quoted in inches while the sidewall is derived in millimetres, so the two must be reconciled before they can be added. Sidewall height is width multiplied by the aspect ratio, and because a wheel has a sidewall at the top and the bottom, that height counts twice:

Overall diameter (mm) = (width × aspect ÷ 100) × 2 + (rim × 25.4)

For a 160/60/17: the sidewall is 160 × 0.60 = 96 mm, the rim is 17 × 25.4 = 431.8 mm, and overall diameter is 96 × 2 + 431.8 = 623.8 mm. Rolling circumference is that figure multiplied by π — roughly 1,960 mm per revolution.

What a Size Change Actually Does

Because the aspect ratio is a percentage of width, changing width alone also changes height. Going from 160/60/17 to 180/60/17 adds 20 mm of width but also 12 mm of sidewall at each end — 24 mm of extra diameter, not zero. Riders who fit a wider rear expecting only a wider contact patch are often surprised to find their gearing effectively taller and their speedo reading further out.

Keeping overall diameter within about 2% of standard is the usual guideline. Beyond that you begin affecting speedometer accuracy, ABS and traction control calibration (both infer wheel speed from rotation), ground clearance, and on some bikes physical clearance at the swingarm or hugger.

Width Is Limited by the Rim, Not Preference

Every rim has a manufacturer-specified range of tyre widths it can safely carry. Fitting a tyre wider than the rim is designed for distorts the carcass into a more rounded profile, which can reduce the contact patch when leaned over — the opposite of what the change was usually intended to achieve. It also changes turn-in behaviour, typically making steering heavier and less precise. A wider tyre is only an upgrade if the rim width supports it.

Checking Your Speedo Against Reality

Most motorcycle speedometers are optimistic from the factory: type approval permits over-reading but not under-reading, so manufacturers build in a margin. That means a tyre change which makes your speedo read lower may simply be moving it closer to the truth. Compare indicated speed against a GPS reading at a steady pace on level road before and after a size change — that comparison isolates what the tyre actually did from the error the speedo already had.