The side of a tire looks like a jumble of letters and numbers, but it is actually a compact data sheet. It tells you the tire’s size, how much weight it can carry, how fast it is rated to go and even the week it was made. Anyone working with cars, from future mechanics to everyday drivers, benefits from reading it correctly, because choosing the wrong tire affects safety, handling and wear. This guide breaks the code down piece by piece.
The size code: 205/55R16
The most prominent marking is the size. Take the common example 205/55R16. Each part has a meaning:
- 205 is the section width in millimeters, measured from one sidewall to the other.
- 55 is the aspect ratio. It means the sidewall height equals 55 percent of the width.
- R stands for radial construction, the standard for modern passenger cars.
- 16 is the wheel (rim) diameter in inches. The tire must match the wheel exactly.
Some tires start with a letter, such as P for passenger car or LT for light truck. A lower aspect ratio means a shorter, stiffer sidewall, which usually gives sharper steering response but a firmer ride. A higher number means more cushioning, which is common on vehicles that prioritize comfort or travel on rough roads.
Load index and speed rating: 91V
After the size you will often see a two- or three-digit number followed by a letter, for example 91V. The number is the load index, a code that corresponds to the maximum weight a single tire can support when properly inflated. It is not the weight itself, so you need a chart to convert it: an index of 91 corresponds to 615 kilograms per tire. The letter is the speed rating, the maximum speed at which the tire is designed to carry its rated load under test conditions.
| Speed symbol | Maximum speed |
|---|---|
| Q | 160 km/h (99 mph) |
| S | 180 km/h (112 mph) |
| T | 190 km/h (118 mph) |
| H | 210 km/h (130 mph) |
| V | 240 km/h (149 mph) |
| W | 270 km/h (168 mph) |
| Y | 300 km/h (186 mph) |
The rule of thumb is simple: never fit a tire with a lower load index or speed rating than the vehicle manufacturer specifies. The recommended values are usually printed on a label in the driver’s door frame or listed in the owner’s manual.
The DOT code and manufacturing date
Look for the letters DOT followed by a long string of characters. The last four digits show when the tire was made. The first two are the week of the year and the last two are the year. For example, 2823 means the tire was produced in the 28th week of 2023. The earlier characters identify the plant and tire details.
This date matters because rubber ages even when the tire is not used. Heat, sunlight and time make it harden and develop tiny cracks. Many tire and vehicle makers advise having tires inspected regularly as they get older and replacing them within a limit that they define, so check the guidance for your tire brand. When buying new tires, it is reasonable to ask for a recent production date rather than ones that sat in a warehouse for years.
Other common markings
Beyond the main code, the sidewall often carries extra information:
- M+S or M&S: mud and snow. It indicates a tread design with better traction than a standard road tire in those conditions.
- 3PMSF (three-peak mountain snowflake): a symbol showing the tire passed a specific snow traction test.
- Max pressure: the maximum inflation pressure the tire can handle. It is not the recommended pressure for your car, which is found on the vehicle’s label.
- Treadwear, Traction and Temperature (UTQG): ratings used in some markets. Treadwear is a comparative number, traction is graded AA, A, B or C, and temperature resistance is graded A, B or C.
- Tread wear indicators (TWI): small bars in the grooves that become flush with the tread when it is worn to the legal minimum.
- Rotation arrows or “Outside” and “Inside”: directional or asymmetric tires must be mounted in the correct orientation.
Practical tips when choosing or replacing tires
Always start from the size recommended by the vehicle manufacturer. Changing width or aspect ratio alters the overall diameter of the wheel assembly, which can throw off the speedometer, the odometer and the work of stability systems. If a different size is truly needed, a qualified technician should confirm that the replacement is compatible.
Check pressure with the tires cold, at least once a month, using the value from the vehicle label. Inspect the sidewalls for bulges, cuts or cracks, and look at the tread for uneven wear, which can point to alignment or suspension issues. Mixing tires with very different characteristics on the same axle is generally discouraged, since it can change how the car behaves in emergency maneuvers.
Conclusion
A tire sidewall is a short manual written in code. Once you know that 205/55R16 91V describes width, profile, construction, rim size, load capacity and speed rating, and that the DOT date reveals the tire’s age, you can compare options with confidence and spot problems early. To build these skills into a career, explore the automotive repair and car maintenance courses on Cursa, which cover suspension, brakes, tires and day-to-day diagnostics.
