Knowing how to read tire size helps you replace tires correctly, compare fitments, and avoid expensive mistakes.
The code on the sidewall looks technical, but each part tells you something useful about width, aspect ratio, construction, wheel diameter, load capacity, and speed rating.
What tire size numbers actually mean
Most passenger tires use a format like P215/65R15 95H.
This sequence is standardized by the Tire and Rim Association and appears on many vehicles in North America and beyond.
Once you understand the pattern, you can read almost any modern tire label.
Here is the basic breakdown:
- P = tire type, often Passenger
- 215 = tire width in millimeters
- 65 = aspect ratio, or sidewall height as a percentage of width
- R = radial construction
- 15 = wheel diameter in inches
- 95 = load index
- H = speed rating
Some tires omit the leading letter or use different prefixes, but the structure is usually similar.
Light truck, temporary spare, and commercial tires may use other markings that follow the same general logic.
How to read tire size step by step
If you want to know how to read tire size quickly, start at the sidewall and identify the sequence of numbers and letters.
The size is usually printed in a format similar to 225/45R17 or LT265/70R16.
Read from left to right.
- Find the width. The first three digits, such as 225, show the section width in millimeters.
- Check the aspect ratio. The next two digits, such as 45, show the sidewall height as a percentage of width.
- Note the construction. The letter R usually means radial, which is the most common tire construction today.
- Identify wheel diameter. The number after the construction letter, such as 17, tells you the rim size in inches.
- Look at load and speed. The final numbers and letters tell you how much weight the tire can carry and how fast it can safely run.
This is not just a technical code.
It directly affects ride comfort, handling, braking, clearance, and fuel economy.
Understanding width, aspect ratio, and wheel diameter
Width
Tire width is measured in millimeters from sidewall to sidewall, though the measurement is taken on an approved rim width.
A 225 tire is wider than a 205 tire, which may improve grip but can also increase rolling resistance and require more space inside the wheel well.
Aspect ratio
The aspect ratio is the sidewall height expressed as a percentage of the width.
A lower number, such as 40 or 45, usually means a shorter sidewall and sportier handling.
A higher number, such as 65 or 70, usually means a taller sidewall and more cushioning over potholes and rough roads.
Wheel diameter
The wheel diameter must match exactly.
A 17-inch tire fits a 17-inch wheel only; it cannot be mounted on a 16-inch or 18-inch rim.
This is one of the easiest ways to eliminate incompatible options when shopping for replacements.
What the load index and speed rating tell you
After the size, the load index and speed rating matter just as much.
These values are often overlooked, but they are essential for safety and legality.
Load index
The load index is a numerical code that corresponds to a maximum load a tire can support when properly inflated.
For example, a load index of 95 means a specific weight capacity per tire according to standardized tables.
When replacing tires, match or exceed the original load index unless the vehicle manufacturer says otherwise.
Speed rating
The speed rating is a letter such as S, T, H, V, W, or Y.
It indicates the tire’s tested speed capability under controlled conditions.
Higher letters generally mean higher speed capability and, in many cases, firmer handling characteristics.
For everyday driving, the rating should meet or exceed the original equipment recommendation.
How to tell if a tire fits your vehicle
Reading the tire size is only the first step.
A tire can look correct on paper and still be wrong for your vehicle.
Check the owner’s manual, the driver-side door placard, and the current tire sidewall before buying.
- Match the recommended size. This is the safest choice for most drivers.
- Verify load and speed specifications. Do not downgrade these ratings without understanding the tradeoffs.
- Check clearance. Larger tires may rub on suspension parts, brake components, or fender liners.
- Review wheel width requirements. Tires are designed for a range of approved rim widths.
If you change tire size, you may also affect the accuracy of the speedometer, odometer, ABS calibration, and traction control system behavior.
Even a small change in overall diameter can have noticeable consequences.
Common tire size formats you may see
Not every tire label looks the same.
Different vehicle categories use different prefixes and markings.
- P-metric: Common on passenger vehicles, such as P215/65R15
- LT: Light truck tires, such as LT265/70R16
- ST: Special trailer tires, such as ST225/75R15
- Temporary spare: Often marked with a T, such as T125/70D16
You may also see additional information such as M+S for mud and snow, 3PMSF for three-peak mountain snowflake certification, or run-flat markings from manufacturers like Bridgestone, Michelin, Goodyear, Pirelli, Continental, and Hankook.
How metric tire sizes differ from flotation sizes
Some off-road and light truck applications use flotation sizing, such as 31×10.50R15.
This format reads differently from metric sizing.
- 31 = approximate overall tire diameter in inches
- 10.50 = section width in inches
- R = radial construction
- 15 = wheel diameter in inches
Flotation sizes are common on certain trucks, SUVs, and modified vehicles.
If your vehicle came from the factory with metric sizes, do not assume a flotation equivalent will fit without checking clearance and ride height changes.
Where to find the tire size on your vehicle
The easiest place to find the correct size is the tire sidewall itself.
You can also check:
- Driver-side door jamb placard
- Owner’s manual
- Fuel door sticker on some vehicles
- Manufacturer tire information label
Keep in mind that the size on the vehicle may differ from the size currently mounted if a previous owner changed the wheels or tires.
That is why it helps to compare the sidewall, the placard, and the manual before purchasing replacements.
Why tire size matters for performance and safety
Choosing the right tire size affects more than appearance.
Tire dimensions influence acceleration, cornering, braking distance, road noise, and cabin comfort.
In many vehicles, the original tire size was selected to balance these factors with steering feel and fuel efficiency.
Replacing tires with the wrong dimensions can lead to:
- Rubbing or interference
- Incorrect speedometer readings
- Poor handling balance
- Reduced load-carrying capacity
- Potential safety system issues
That is why mechanics, tire retailers, and vehicle manufacturers all recommend matching the original specifications as closely as possible unless you have a specific reason to change them.
Quick checklist for reading tire size correctly
- Read the full sidewall code, not just the width
- Confirm tire type prefix, such as P, LT, or ST
- Match wheel diameter exactly
- Verify load index and speed rating
- Compare the size to the door placard and owner’s manual
- Check overall diameter if you are considering a different size
Once you know how to read tire size, you can shop more confidently and understand what those sidewall markings really mean.
The code is short, but it contains the key information needed to choose a tire that fits, performs properly, and supports safe driving.