Motorcycle Tire Sizes Explained (120/70-ZR17)
Reviewed by CrankDex editorial
A size like 120/70-ZR17 looks like a code, but every part of it means something specific: width, sidewall height, construction, and rim diameter. Here's how to read it.
Motorcycle tire sizes follow a standard format that packs several pieces of information into one string of numbers and letters. Once you know what each part represents, a size like 120/70-ZR17 stops looking cryptic and starts reading like a spec sheet.
Width: the first number
The first number, 120 in this example, is the tire's section width in millimeters, the distance across the tire at its widest point when mounted and inflated to spec. A bigger number means a wider tire.
Aspect ratio: the second number
The number after the slash, 70 here, is the aspect ratio, expressed as a percentage of the width. A 70 aspect ratio means the sidewall height is 70 percent of the 120mm width. Lower aspect ratio numbers mean a shorter sidewall, generally associated with sportier handling; higher numbers mean a taller sidewall, generally associated with more cushion and comfort.
Construction: the letter
The letter before the rim size, Z in ZR17, indicates construction type and speed capability. R specifically indicates radial construction. A Z in front of the R (as in ZR) is a speed rating designation historically used for tires rated above 149 mph, now commonly paired with a separate explicit speed rating elsewhere in the tire's full specification. Some tires use B for bias-ply construction instead of R. Matching construction type to what your bike was designed for matters for handling, so this isn't a detail to skip past.
Rim diameter: the last number
The final number, 17 in this example, is the rim diameter in inches, the size of wheel the tire is designed to mount on. This has to match your actual wheel size; it's not adjustable or approximate the way some other tire dimensions can flex slightly between brands.
Load and speed ratings
Beyond the core size, tires also carry a load index and speed rating, usually shown as a number and letter right after the size, like 58W. The load index reflects the maximum weight the tire is rated to carry, and the speed rating reflects the maximum sustained speed it's rated for. Both matter for safety and are worth matching to, or exceeding, your bike's original specification rather than going lower to save money.
Front and rear are usually different
Don't assume one size covers the whole bike. Rear tires are typically wider than front tires since they handle more of the power delivery and braking load, so most motorcycles specify two different sizes, one for front and one for rear. Always check your manual or a fitment tool for each position separately.
Matching your OEM size
The size printed on your current tire's sidewall, or listed in your owner's manual, is the safest starting point for a replacement. The whole size, width, aspect ratio, construction, and rim diameter, together determine how the tire behaves on your specific bike, so it's worth matching all of it rather than picking a size that only shares the rim diameter.
Browse motorcycle tires on Amazon (we may earn a commission, at no extra cost to you), or look up your bike to confirm the exact tire size before you buy.
Frequently asked
What does the first number in a tire size mean?
It's the tire's section width in millimeters, measured across the widest point of the tire from sidewall to sidewall. In a size like 120/70-ZR17, that's the 120.
What does the second number mean?
That's the aspect ratio, the tire's sidewall height expressed as a percentage of the section width. In 120/70-ZR17, the 70 means the sidewall height is 70 percent of the 120mm width. A lower number means a shorter, stiffer-feeling sidewall.
Can I put a different size tire on my bike than what came stock?
It's generally safest to stick with the size specified in your owner's manual, since the bike's handling, speedometer accuracy, and clearance around the swingarm or fender were all set up around that size. Deviating from it is a decision worth researching carefully rather than guessing.
Do the front and rear tire need to match sizes?
No, and on most motorcycles they're deliberately different. Rear tires are typically wider than front tires to handle more of the power and braking load, so always match each position (front and rear) to its own specified size rather than assuming one size fits both.
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