Motorcycle Tyre Sizes Explained UK | Read Sidewall Codes

Motorcycle tyre sizes in the UK are decoded from the sidewall markings, which specify width, aspect ratio, construction, rim diameter, load index, and speed rating.

For the full breakdown, see our best Motard Tyres guide.

Every motorcycle tyre sold in the UK carries a code on its sidewall telling you everything about its size and capability. Once you learn to read it, you can check fitment, load capacity, and UK legal compliance. The code follows a consistent pattern across metric, inch, and older sizing systems. Getting the size right matters for handling, safety, and legality on UK roads.

Breaking Down The Sidewall Code

The most common format on modern UK road motorcycles is metric: width / aspect ratio – construction – rim diameter, with load index and speed symbol often appended. A code reading 180/55ZR17 means the tyre is 180 mm wide, sidewall height equals 55% of that width, Z-rated (over 150 mph), and fits a 17-inch rim. The speed rating or construction letter can sit before the rim diameter rather than after — 120/70ZR17 and 120/70R17Z mean the same thing but follow different conventions.

Older motorcycles and some off-road or enduro tyres use an inch-based system, e.g., 2.75-18, where the first number is width in inches and the second is rim diameter in inches. Michelin’s UK motorcycle tyre size guidance provides correspondence tables between metric and inch sizes for enduro and motocross tyres, but these are approximations — the two systems do not map one-to-one.

The sidewall also carries a DOT date code: a four-digit number where the first two digits are the week of production and the last two are the year. For example, 2312 means the 23rd week of 2012. Tyres older than five or six years need close inspection before use, even if the tread looks fine.

Finding The Right Size For Your Bike

Before buying a replacement, check both the markings on the current tyre and the size in your motorcycle’s owner or service manual. The manufacturer’s recommendation is the safest reference because it accounts for handling, suspension geometry, and clearance. The load index and speed rating must meet or exceed the original specification — fitting a lower-rated tyre compromises safety at higher speeds or under load and may cause an MOT failure.

If you are fitting motard or supermoto-style tyres to a compatible bike, the correct size directly affects steering response and stability. For a tested selection that fits standard sizes, see our roundup of the best motard tyres for your machine. Never assume metric and inch sizes with matching numbers are interchangeable — always check a correspondence table before substituting.

UK MOT Requirements For Motorcycle Tyres

UK law sets specific standards that the MOT test checks. The table below summarises the key pass-fail criteria from the official MOT inspection manual.

Common MOT failures include worn tread below the legal minimum, structural damage such as bulges or exposed cords, and cuts revealing underlying cords. Even a small cut can fail if probing opens it beyond allowed depth. Checking these areas before the test saves time and avoids a retest fee.

Stick to the pressures in your owner’s manual for road use.

The safest approach is straightforward: stick with the manufacturer’s recommended size, read the sidewall code before buying any replacement, and inspect tread depth and condition regularly. The MOT standards above give a clear reference for legality, and the date code helps avoid riding on tyres older than their tread suggests.

FAQs

Where do I find my motorcycle tyre size?

The tyre size is printed or embossed on the sidewall of every motorcycle tyre. Look for a code such as 120/70ZR17 or 4.00-18, usually followed by a load index and speed symbol. Your owner’s manual also lists the correct size for both front and rear wheels as specified by the manufacturer.

Can I fit a wider tyre than standard on my motorcycle?

Fitting a wider tyre may affect handling, clearance, and suspension geometry. The manufacturer’s recommended size is the safest choice. Size conversion tables are approximations, and changing sizes without verifying fitment can cause steering issues or contact with the swingarm or mudguard. Always check actual clearance before fitting a non-standard size.

How do I read the tyre date code?

The DOT date code is a four-digit number on the sidewall. The first two digits indicate the week of production and the last two indicate the year. For example, 2312 means the 23rd week of 2012. This helps determine tyre age, useful when buying second-hand or old-stock tyres.