Motorcycle Tyre Size Guide: How to Read the Numbers

Motorcycle tyre sizes look like a code at first glance. 120/70 ZR17 (58W) is not a random string of letters and numbers. Each part tells you something specific about the tyre, and once you learn to read them you can match a new tyre to your bike confidently. This guide breaks down the format, explains tubeless versus tube type, covers how front and rear tyres differ, and shows how to find your bike's correct size.

Decoding the size format

The standard motorcycle tyre size format reads like this: 120/70 ZR17 58W. Reading left to right, here is what each piece means.

Tyre width (120)

The first number is the section width of the tyre in millimetres, measured from sidewall to sidewall when the tyre is mounted at the correct pressure. A 120 width tyre is 120 mm wide. Common front widths are 100, 110, 120 and 130. Common rear widths are 150, 160, 170, 180, 190 and 200.

Aspect ratio (70)

The second number is the aspect ratio, expressed as a percentage of the width. An aspect ratio of 70 means the tyre's sidewall height is 70 percent of its width. So a 120/70 tyre has a sidewall height of 84 mm (70 percent of 120).

Lower aspect ratios mean shorter sidewalls and a stiffer feel under cornering. Sport bikes typically run aspect ratios of 50, 55 or 60. Cruiser and touring bikes often run 70 or 80. Off-road tyres can sit at 90 or 100.

Construction (ZR)

The letter or letters between the aspect ratio and rim diameter describe the tyre's internal construction.

• R: radial construction. Plies run perpendicular to the direction of travel. Modern, common on most road tyres above 130 km/h speed ratings.
• B: belted bias construction. A halfway design between bias and radial.
• No letter (or D): bias-ply (cross-ply) construction. Older style, still used on some cruiser and dirt tyres.
• ZR: radial construction with a Z speed rating (over 240 km/h capability).

Rim diameter (17)

The third number is the wheel rim diameter in inches. A 17 means the tyre fits a 17-inch rim. This number is not negotiable. You cannot fit a 17-inch tyre to an 18-inch rim. Common rim sizes for road bikes are 17 and 18 inches. Off-road bikes use 18, 19 and 21 inch rims. Cruisers often run 16, 17 or 21 inch front and 15 or 16 inch rear.

Load index (58)

The two-digit number after the rim size is the load index. Each number corresponds to a maximum load the tyre can carry at the rated speed. A load index of 58 means 236 kg. A load index of 73 means 365 kg. Tyre manufacturers publish load index tables online, and the number on the tyre is also stamped on a label inside the chain guard or under the seat on most bikes.

Speed rating (W)

The letter at the end is the speed rating. Common motorcycle ratings:

• H: up to 210 km/h.
• V: up to 240 km/h.
• W: up to 270 km/h.
• Y: up to 300 km/h.

You can fit a higher-rated tyre than your bike calls for, but you should not fit a lower-rated one. The OEM specification on your bike is a minimum, not a target.

Tubeless vs tube type

Two more letters you will see on the sidewall:

• TL: tubeless. The tyre is designed to seal directly to the rim without an inner tube. Most modern alloy and cast wheels run tubeless setups.
• TT: tube type. The tyre requires an inner tube to hold air. Spoked wheels traditionally need TT tyres unless the rim is sealed for tubeless use.

Mismatching is a common error. Fitting a tube to a tubeless tyre is technically possible but can cause heat build-up under load. Fitting a tubeless tyre to a spoked rim without sealing the spokes will leak.

Front vs rear tyres

Front and rear motorcycle tyres are not the same. Fronts are narrower, run a more rounded profile and are tuned for steering response and braking grip. Rears are wider, run a flatter centre tread profile and are tuned for traction under acceleration and load.

You should never run a front tyre on the rear or vice versa. Tread direction also matters. Look for the rotation arrow on the sidewall and fit the tyre so it spins in the indicated direction.

On most bikes the front and rear sizes are different. A typical sport bike might run a 120/70 ZR17 front and a 180/55 ZR17 or 190/55 ZR17 rear. A typical cruiser might run 130/90 16 front and 170/80 15 rear. A typical adventure bike often runs 110/80 19 front and 150/70 17 rear.

Where to find your bike's correct size

Three reliable sources, in order of preference:

1. The chain guard or swingarm sticker. Most modern bikes have a tyre information label here listing the OEM tyre sizes and recommended pressures.
2. The owner's manual. The maintenance section will list approved tyre sizes.
3. The current tyres on the bike. Read the size off the sidewall as long as you trust whoever fitted them last.

If you change rim size (for example fitting wider aftermarket wheels) the tyre size needs to change with it. Speak to a qualified tyre fitter before changing tyre sizes outside OEM specification.

Common road bike sizes

To give you a feel for what fits where, these are typical OEM tyre sizes for popular bike categories:

• Sport bikes: 120/70 ZR17 front, 180/55 ZR17 or 190/55 ZR17 rear.
• Naked bikes: 120/70 ZR17 front, 160/60 ZR17 to 180/55 ZR17 rear.
• Sport touring: 120/70 ZR17 front, 180/55 ZR17 or 190/55 ZR17 rear.
• Adventure bikes (street oriented): 110/80 R19 front, 150/70 R17 rear.
• Cruisers: 130/90 16 or 100/90 19 front, 170/80 15 or 150/80 16 rear.
• Scooters: 110/70 12 or 120/70 12 front, 130/70 12 rear.

These are starting points. Always confirm against your specific year and model.

When to replace

Three signs a tyre is at end of life:

• Tread wear indicators (TWI): small raised rubber bars in the tread grooves. When the tread surface is level with the bars, the tyre is at minimum legal depth. In Australia the minimum is 1.5 mm.
• Age: rubber compounds harden over time. The DOT date code on the sidewall is a four-digit number where the first two digits are the week and the last two are the year (for example 1623 means week 16 of 2023). Most manufacturers recommend replacement after five to seven years regardless of tread depth.
• Visible damage: sidewall cracks, bubbles, embedded objects or unusual wear patterns mean the tyre needs inspection by a qualified fitter.

Browsing the right size

The Star Cycle Gear motorcycle tyres collection covers sport, touring, dual sport, dirt and cruiser tyres across the size ranges above. Brands stocked include Pirelli and Goodride at the time of writing. If you are buying for an ATV or quad bike, the ATV tyres collection is the place to start instead, since ATV sizing follows a different format. The full tyre range covers both.

Once you know your front and rear OEM sizes, filtering by size on the collection page will narrow the catalogue to options that fit your bike.