To understand tire size and specs, you can use the code printed on the sidewall, like the example in the image: 225/40 R18 94Y.
Tire width (225)
The first number (225) is the tire’s width in millimeters.
It measures the distance from one sidewall to the other when the tire is properly mounted and inflated.
A higher number means a wider tire, which usually gives more grip but can increase fuel consumption.
Aspect ratio (40)
The second number after the slash (40) is the aspect ratio or profile.
It is the sidewall height expressed as a percentage of the tire’s width.
Here, the sidewall height is 40% of 225 mm, so this is a low-profile, sportier tire.
Radial construction (R)
The letter after the aspect ratio (R) tells you the construction type.
R means radial, which is the modern standard where the internal layers run radially across the tire.
Almost all passenger car tires today are radial, so you’ll usually see an R here.
Rim diameter (18)
The number after the construction letter (18) is the rim diameter in inches.
It shows the wheel size this tire fits, so this one mounts on an 18‑inch rim.
You must match this exactly to your wheel size; 18-inch tires do not fit 17‑inch rims.
Load index (94)
The next number (94) is the load index.
It is a code that corresponds to the maximum weight one tire can carry at the correct pressure.
To know the exact kilograms, you look up “load index table” and match the number to its weight rating.
Speed rating (Y)
The final letter (Y) is the speed rating.
It indicates the maximum speed the tire can safely handle under the correct load and pressure.
Different letters (T, H, V, W, Y, etc.) represent different top speeds, so you should choose a rating that meets or exceeds your car manufacturer’s recommendation.
Use the banner image as a visual guide: follow the white lines from each part of the code “225/40 R18 94Y” on the sidewall to see exactly which number or letter corresponds to width, ratio, construction, rim size, load index, and speed rating.