Motorcycle tire sizes
Motorcycle tire sizes leave out the one number that decides speedometer, gearing and clearance: overall diameter.
Motorcycle tires
A modern motorcycle tire size such as 180/55 ZR 17 (73 W) reads as four separate pieces of information. The 180 is the nominal section width in millimeters. The 55 is the aspect ratio: the sidewall height as a percentage of that width, so 55% of 180 mm is 99 mm of sidewall.
The 17 is the rim diameter in inches, and the letters before it describe construction and speed capability. The figures in brackets are the load index and speed symbol. They state the tire's maximum load and speed.
Overall diameter is not printed anywhere: you calculate it as the rim diameter plus twice the sidewall height. Nominal tire diameter supports an initial gearing comparison. Actual rolling circumference, approved fitment and physical clearance require separate checks; a GPS speed reading does not change because wheel diameter changes.
Reading the size, number by number
Nominal width in millimeters at the widest point of the inflated tire, measured on the manufacturer’s specified measuring rim. Fit the same tire to a narrower rim and it comes out narrower and more pointed. Fit it to a wider one and it flattens; this is why a rim width range is published for every tire size.
Sidewall height as a percentage of section width. A lower number is a shorter sidewall. Shorter sidewalls are stiffer and give more precise steering with less compliance.
Because it is a percentage of width, changing the width alone also changes the sidewall height in millimeters and therefore the overall diameter.
Construction and speed capability. R marks a radial, B a belted bias, and a plain dash a bias-ply tire. ZR historically indicated a tire rated above a defined high speed, and on modern tires the speed symbol in the service description is the precise statement.
Bead seat diameter in inches. This is the only number that is not negotiable at all: the tire either seats on that rim diameter or it does not. There is no adaptation.
Load index and speed symbol. The load index is a code corresponding to a maximum load in the industry load tables, and the speed symbol to a maximum sustained speed. Fitting below either of the original figures reduces a designed margin.
The older systems you will still meet
Two earlier size systems remain on older machines and on some cruiser and classic tires. The alphanumeric system uses codes such as MT90-16, where the letters describe a width class instead of a measurement. The inch system uses sizes such as 4.00-18, where the first number is the approximate section width in inches.
Both are convertible to metric only approximately, and conversion charts disagree at the edges because the underlying systems do not map cleanly. Where a machine specifies one of these, the safest approach is a tire marked in that system , not a metric equivalent chosen from a chart.
| System | Example | What the first number is | Where you meet it |
|---|---|---|---|
| Metric | 180/55 ZR 17 | Section width in millimeters | Effectively all modern tires |
| Alphanumeric | MT90-16 | A letter code for a width class, not a measurement | Cruiser and older touring fitments |
| Inch | 4.00-18 | Approximate section width in inches | Classic machines, some trail tires |
The metric size, load index and speed symbol are all standardized markings, so they mean the same thing from any manufacturer. The actual molded dimensions vary by a few millimeters between makers. Only where clearance is already tight does it matter.
Why a wider rear tire is not automatically an upgrade
A motorcycle tire's profile, the curve of the tread across its width, is designed for a specific rim width range. Fitting a wider tire to the original rim pinches that profile into a more pointed shape than intended. The machine then falls into corners more quickly at small lean angles and runs out of contact patch differently at large ones, which is felt as a change in steering character and not as more grip.
At the same aspect ratio, a wider nominal tire is also taller. Using the nominal-size formula, 190/55-17 is 11 mm larger in overall diameter than 180/55-17. Actual dimensions, approved rim widths, clearance and electronic-system compatibility still need the machine and tire manufacturers’ fitment information.
Pressure, and why the number on the tire is the wrong one
The pressure molded into a tire's sidewall is a maximum for the tire, not a recommendation for your machine. The correct pressure comes from the machine manufacturer, usually on a sticker on the swingarm or under the seat, and in the manual; it is often different front and rear, and different again for a loaded machine or a passenger.
Pressure changes more than comfort. Inflation pressure affects load support, carcass deflection and heat.
Uneven wear can have several causes and does not prove a particular pressure fault; use the machine’s cold-pressure specification and inspect the tire. Check pressure cold using the machine manufacturer’s specified conditions. Do not bleed a hot tire down to its cold-pressure value.
References
- Federal rule FMVSS No. 119: New pneumatic tires for motor vehicles with a GVWR of more than 4,536 kg and motorcycles: U.S. Government Publishing Office (eCFR), 49 CFR § 571.119.
- S
- Set by a published standard; The metric size, load index and speed symbol are all standardized markings, so they mean the same thing from any manufacturer. The actual moulded dimensions vary by a few millimeters between makers. Only where clearance is already tight does it matter.
Why a wider rear tire is not automatically an upgrade
A motorcycle tire's profile, the curve of the tread across its width, is designed for a specific rim width range. Fitting a wider tire to the original rim pinches that profile into a more pointed shape than intended. The machine then falls into corners more quickly at small lean angles and runs out of contact patch differently at large ones, which is felt as a change in steering character and not as more grip.
The wider tire is also taller, because the sidewall height is a percentage of width. A 190/55 on a 17-inch rim is roughly a centimetre larger in overall diameter than a 180/55, which raises the rear of the machine slightly, steepens the steering angle, and slows the speedometer. None of that is catastrophic and all of it is a change worth making deliberately instead of by accident.
Motorcycle tire sizes questions
What does 180 55 ZR 17 mean?
The 180 is nominal section width in millimeters, the 55 is the sidewall height as a percentage of that width, the ZR describes radial construction with high speed capability. The 17 is rim diameter in inches. Overall diameter is not stated and has to be calculated: rim diameter plus twice the sidewall height. Here that is about 24.8 inches.
How do I work out overall tire diameter from the size?
Multiply the section width by the aspect ratio to get sidewall height in millimeters, convert to inches by dividing by 25.4, double it because there is sidewall above and below the rim, and add the rim diameter. For 180/55 R17: 180 × 0.55 = 99 mm, or 3.9 in. Doubled is 7.8 in. Add 17 and you get 24.8 in.
What is the number in brackets after the tire size?
It is the service description: a load index followed by a speed symbol. The load index is a code that corresponds to a maximum load in published tables, and the speed symbol to a maximum sustained speed. Both are ratings for the tire, and fitting below the original figures reduces a margin the machine was designed around.
Is the pressure on the tire sidewall the pressure I should use?
No. That figure is a maximum for the tire itself. The pressure to run comes from the machine manufacturer, usually on a sticker on the swingarm or under the seat and in the manual; it commonly differs front to rear and changes for a loaded machine or a passenger. Check it cold, because a warm tire reads several psi high.
Can I fit a wider rear tire on the standard rim?
Often it physically fits and it is rarely an improvement. A wider tire on an unchanged rim is squeezed into a more pointed profile than it was designed for, which changes how the machine turns in; it is also taller because the sidewall is a percentage of the width. Check the published rim width range for the size before deciding.
What is the difference between radial and bias ply motorcycle tires?
The angle of the carcass plies. A radial has plies running across the tire with a belt under the tread, which gives a more flexible sidewall and a stiffer tread, better heat resistance and more precise steering. A bias-ply has crossed plies, a stiffer sidewall and more load-carrying character. Machines are developed around one or the other; mixing types front and rear is generally not recommended.
What does MT90 16 mean on an older motorcycle?
It is an alphanumeric size. The letters are a width class code, not a measurement, the 90 is an aspect ratio. The 16 is rim diameter in inches. Conversion charts to metric exist and disagree at the edges, because the systems do not map cleanly. Where a machine specifies an alphanumeric size, a tire marked in that system is safer than a converted equivalent.
Do I have to replace both tires at once?
Not necessarily, but front and rear from one range are developed as a matched pair with complementary profiles. Replacing one leaves you with a mismatch in profile and in wear state, and the machine steers differently as a result. Where the remaining tire is more than half worn or several years old, replacing both is usually the better decision.
What does a speed symbol rate?
A maximum sustained speed the tire is certified to run at under specified load and inflation conditions; it is a laboratory rating and not a recommendation, and it assumes correct pressure and an undamaged tire. Fitting a lower speed rating than the original reduces a margin that was part of the machine’s design case.
Why does my new tire feel like the bike steers differently?
Because profile shape, carcass stiffness and tread depth all change how the machine turns, and a new tire has full depth and a fresh profile. Some of it is a genuine difference between tire models and some is simply new rubber. Give it the run-in the manufacturer recommends before judging, and check pressures cold first.
What is the run in period on a new motorcycle tire?
A distance of gentle riding, typically stated by the tire manufacturer, to let the mould release agent wear off the surface and the tire bed onto the rim. The specific distance comes from the tire maker instead of from any standard. The practical point is that a brand-new tire has less grip than the same tire after a hundred miles.
Does tread depth matter as much on a motorcycle as on a car?
It matters for water clearance in the same way; it matters differently because a motorcycle uses the shoulders of the tire in corners while wearing the center on straight roads. A tire can be legal in the center and finished at the shoulder, or squared off in the center with plenty of shoulder tread left. Inspect across the whole width, not just at the center groove.
Do I need to rotate motorcycle tires?
No: tire rotation is a car concept that does not carry over. A motorcycle's front and rear tires are different sizes, different profiles and often different compounds, developed as a matched pair for their specific position. There is no valid position to move either one to. Each is replaced on its own wear, not swapped front-to-rear.
Do I need a wheel alignment on a motorcycle?
The equivalent job exists but it is not the four-wheel geometry a car gets. It is rear wheel alignment, making sure the rear wheel tracks straight in line with the front, not pointing slightly off to one side, which a chain or belt final drive can pull out of true as it is adjusted for slack. Most swingarms carry index marks for this at each chain adjuster.
A UTV's tie-rod toe alignment (see the ATV and UTV tire page) is a different mechanism entirely.
Where the figures and rules come from
Standard
Year Book: standardized tire, rim and valve dimensions, load and inflation tables : The Tire and Rim Association.
Standard
Standards Manual: tire and rim dimensional and load/inflation standards : European Tyre and Rim Technical Organisation.
Federal rule
FMVSS No. 119: New pneumatic tires for motor vehicles with a GVWR of more than 4,536 kg and motorcycles : U.S. Government Publishing Office (eCFR), 49 CFR § 571.119.
Tire size change calculator; Diameter, revolutions per mile, speedometer error and ride height, from the size.
- DOT date codes and how old that tire is
- Age matters independently of tread depth, and the date is moulded into the sidewall.
- Wheel bolt patterns and offset
- Rim width has to suit the tire, and it changes the profile you end up with.
- Fitment: the four questions in order
- Attach, clear, carry, change, and which of them a size can answer.