Tire size converter
What does 120/70-17 mean, and is there an alternative? Decode any size, compare two sizes with speedo effect, or load the stock sizes of your motorcycle.
Find motorcycle tire sizes
Search a model — its stock front and rear sizes fill in automatically from our spec data.
Decode a tire size
Examples: 120/70-17 · 180/55 ZR17 · 90/90-10 · 3.50-19
Old inch-style size — a ~100% aspect ratio is assumed, figures are approximate.
Could not read this size — check the format (e.g. 120/70-17).
Compare an alternative size
Diameter is only one side of the story: also confirm the rim-width range on the tire maker's chart, clearance to fender/swingarm/chain, and never go below the stock load and speed index.
Frequently asked questions
What does 120/70-17 mean?
Three numbers: 120 is the tire width in millimetres, 70 is the aspect ratio (sidewall height as a percentage of the width — here 84 mm), and 17 is the rim diameter in inches. Letters in between tell the construction and speed rating: ZR17 means radial with a Z speed rating.
Can I fit an alternative tire size?
Within limits. The safe rule of thumb is to keep the overall diameter within about 1.5-2% of stock, stay inside the rim-width range the tire maker specifies, and check clearance to the fender, swingarm and chain. Big diameter changes upset the speedometer, ABS behaviour and steering geometry.
What does the aspect ratio change?
A lower profile (70 → 60) means a shorter, stiffer sidewall: sharper steering response but less comfort and a smaller overall diameter. A taller profile does the opposite. Because it also changes the diameter, compare the full size — not just the profile number.
What are the load and speed indexes — can I go lower?
The number is the load index (73 = 365 kg per tire) and the letter is the speed rating (W = up to 270 km/h). Never fit a tire below the motorcycle manufacturer's minimum — it is a safety and legal/insurance issue. Going higher is fine.
Can front and rear tires be different brands?
It works in an emergency but is not recommended: front and rear are designed as a pair with matching profiles and compounds. Mixing very different characters (a sporty front with a touring rear) can make the bike vague or nervous, especially in the wet.
How do old inch sizes like 3.50-19 translate?
Vintage sizes give the width in inches with roughly a 100% aspect ratio: 3.50-19 is about 89 mm wide on a 19-inch rim. The converter above accepts these and approximates the metric equivalent — useful when putting modern rubber on a classic.
Does a wider tire always grip better?
No. On a rim narrower than the tire is designed for, a wide tire pinches into a peaky profile, steers worse and runs hotter. Extra width also adds weight and slows steering. Grip comes from the right size on the right rim with the right pressure, not from width alone.
Will my speedometer read wrong after a tire size change?
If the new overall diameter differs, yes — by the same percentage. A larger diameter makes the speedo read low (you go faster than displayed). Use the comparator above: it shows what your real speed is when the display says 100 km/h.