Reading a Tyre Sidewall: Size, Load Index and Speed Rating
Every number moulded into a tyre sidewall means something specific, and two of them decide whether the tyre is legal to fit to your car at all.
A marking like 225/40 R18 92Y gives width, profile, construction, rim size, then load index and speed rating. The load index and speed rating are the two you must not drop below your car’s requirement.
Tyre sidewalls look like a wall of codes. Almost all of it is standardised, so once you can read one tyre you can read any of them. Here is what each block actually tells you.
The size block
Take 225/40 R18. The first number is section width in millimetres, measured at the widest point of the inflated tyre. The second is the aspect ratio, and it is a percentage rather than a measurement: a 40 profile means the sidewall height is 40 percent of the 225 mm width, so about 90 mm. That is why fitting a wider tyre on the same rim also makes the sidewall taller unless you drop the profile to compensate. R means radial construction, which is what almost every road tyre uses. The last number is the rim diameter in inches, so this tyre only fits an 18 inch wheel. Mixing metric and imperial in one code is a historical quirk, not a mistake.
Load index and speed rating
The 92Y on the end is the important part. 92 is the load index, a coded figure rather than a weight: it corresponds to 630 kg per tyre. Y is the speed rating, and it means the tyre is rated to 300 km/h. Common ratings below it are H at 210, V at 240 and W at 270. Your vehicle handbook or the placard in the door shut gives the minimum load index and speed rating for the car, and going below either is the one substitution that genuinely matters. XL or Extra Load on the sidewall means a reinforced casing with a higher load index at a higher inflation pressure, which is why heavy cars often specify it.
The rest of the markings
A four digit DOT code gives the week and year of manufacture, so 3225 means week 32 of 2025. Age matters on a tyre that has done low mileage, because rubber hardens whether or not it is used. M+S indicates a mud and snow tread pattern, which is a self declared marking. The three peak mountain snowflake symbol is the tested winter one, and that is the symbol that counts where winter tyres are required. Directional tyres carry a rotation arrow and asymmetric ones are marked outside and inside, both of which are easy to fit wrong. Tyres sold for the US market also carry UTQG treadwear, traction and temperature grades.
The conditions that matter
- ✓Width, profile and rim diameter can be changed if the rolling circumference stays close to standard.
- !Load index and speed rating must meet or exceed what the vehicle specifies.
- !The three peak mountain snowflake symbol, not M+S, is the tested winter marking.
General guidance only. Details vary by exact vehicle and change over time; confirm with a reputable specialist before modifying.