Understanding tire size starts with the code molded into the sidewall, but reading that code is only half the job. A marking such as 225/60R17 99H describes a tire’s nominal width, profile, construction, wheel diameter, load index and speed symbol. It doesn’t confirm that the tire is approved for your vehicle. For that, compare the full specification with the driver-door placard, owner’s manual or manufacturer fitment information. This tire size explained guide shows how to decode the markings, check a replacement size and avoid choosing a tire simply because it fits the wheel.
Read the main size from left to right. In 225/60R17, each part describes a different dimension or construction feature. The numbers are nominal specifications, so they don’t promise that every tire with this code will measure exactly the same once mounted and inflated.
| Marking | What it means | What to remember |
|---|---|---|
| 225 | Nominal section width in millimetres | The width is measured across the tire’s sidewalls, not just the tread. Actual dimensions can vary by model and measuring rim. |
| 60 | Aspect ratio: sidewall height as a percentage of section width | Here, the nominal sidewall height is 60% of 225 mm, or about 135 mm. |
| R | Radial construction | Most passenger-car tires use radial construction. R is not a wheel-size abbreviation. |
| 17 | Wheel diameter in inches | The tire must be designed for a 17-inch wheel. This is not the tire’s total outside diameter. |
| 99 | Load index | This number corresponds to a maximum load per tire in a standardised load-index table. It is not the weight in kilograms or pounds. |
| H | Speed symbol | This letter corresponds to a defined speed capability under specified test conditions. It does not make that speed safe or legal on the road. |
The first number is the tire’s nominal section width in millimetres. A 225-size tire is designed to be about 225 mm wide across its sidewalls when fitted to the specified measuring rim. The tread itself is narrower, and the finished tire’s dimensions can differ between manufacturers and tire models.
The aspect ratio describes sidewall height relative to width. For a 225/60 tire, the nominal sidewall height is 60% of 225 mm, or 135 mm. A smaller aspect-ratio number generally means a shorter sidewall for a given width, while a larger number generally means a taller one. That difference can affect ride feel, steering response and how much sidewall is available to cushion impacts, but the tire’s construction and vehicle setup matter too.
The R in 225/60R17 means radial construction. The number that follows, 17, gives the wheel diameter in inches. A 17-inch tire must be mounted on a wheel with a 17-inch rim diameter; a tire that is otherwise close in size won’t safely mount on a different-diameter wheel.
The wheel diameter isn’t the tire’s overall diameter. In this example, the nominal overall diameter is the 17-inch wheel (431.8 mm) plus two sidewalls of about 135 mm each, or approximately 702 mm. That is a calculation from nominal markings, not a guarantee of the mounted tire’s actual measurement.
The size code is often followed by a load index and speed symbol, such as 99H. The load index refers to the maximum load a tire can carry at the conditions specified for its rating. Use the index table supplied by a reliable tire reference or manufacturer to identify the corresponding capacity; don’t treat the index as a direct weight measurement.
The speed symbol indicates a tire’s speed capability under defined test conditions. It is not a recommended driving speed, and it doesn’t override road rules, weather conditions or the vehicle’s limits. When replacing tires, meet the vehicle manufacturer’s required load and speed specifications. If you’re considering a different rating, confirm that it is acceptable for your exact vehicle and intended use before buying.
Some tires also carry markings such as XL or Extra Load, which identify a reinforced load-capacity category. That label does not, by itself, prove that the tire is suitable for a particular vehicle. Check the required size, load index and inflation guidance together.
Use the vehicle’s specifications as the starting point, not just the tire already fitted. A previous owner may have installed a non-standard size, and the sidewall alone can’t confirm that it is an approved fitment.
Sometimes a vehicle manufacturer lists more than one approved size, often for different wheel packages. Those listed alternatives are the safest place to look if you want a different wheel diameter or tire profile. An approved alternative is more than a tire that can be mounted: it is a fitment intended to work with the vehicle’s clearance, load requirements and systems.
Changing size outside the approved options can alter overall tire diameter, clearance, handling and the relationship between wheel rotation and vehicle-speed readings. A larger or smaller overall diameter may also affect how systems that rely on wheel-speed information behave. Depending on the vehicle and fitment, the change can create rubbing at the suspension or bodywork, or leave insufficient clearance under steering or suspension movement.
Wider tires, shorter sidewalls or larger wheels don’t automatically improve performance. A wider setup may change steering feel and can be more vulnerable to damage from road impacts; a shorter sidewall may make the ride firmer. A taller sidewall may provide more cushioning, but any change still needs to meet the vehicle’s fitment and load requirements. The result depends on the complete tire, wheel and vehicle combination.
Before ordering a non-standard size, have a tire professional check the exact vehicle, wheel width, tire dimensions, load and speed ratings, and clearance through the full range of steering and suspension movement. Also verify whether the size is compatible with the vehicle’s tire-pressure monitoring system. “It fits the rim” is not enough to establish safe or correct fitment.
The main size is only one part of a tire’s sidewall. Additional letters and symbols can identify the tire’s intended application or performance characteristics. Read them alongside the vehicle’s requirements rather than using any single marking as a substitute for a fitment check.
Start with the tire placard on the driver’s door frame and the owner’s manual. If the information is missing or you’re unsure whether the current tires are correct, ask the vehicle manufacturer or a qualified tire professional to confirm the approved specification for your exact vehicle.
A higher load index indicates a greater rated load capacity, but it doesn’t automatically make a tire the best choice. The tire still needs the correct size, compatible wheel fitment and suitable characteristics for the vehicle. Follow the manufacturer’s requirements when comparing options.
That depends on the vehicle, the condition of the existing tires and the requirements of its drivetrain. The replacement tires must match the approved specifications, and differences in tread depth or tire circumference can matter on some vehicles. Check the owner’s manual or ask a tire professional before deciding.
No. The code gives nominal dimensions, while the finished size can vary by tire model and manufacturer. If exact clearance or fitment is important, check the specific tire’s technical information and have the complete wheel-and-tire combination assessed.
No. It is a rating established under specified test conditions, not a safe or legal road speed. Choose a tire that meets the vehicle’s required rating, then drive according to road conditions and applicable speed limits.
It can, because overall tire diameter affects how far the vehicle travels with each wheel rotation. The size change may also affect clearance and systems that use wheel-speed data. Confirm that any alternative size is approved for your vehicle rather than relying on an assumed speedometer correction.
Tire size explained in practical terms comes down to two checks: decode the complete sidewall marking, then compare it with the fitment requirements for your vehicle. Match the approved size and required load and speed ratings, and use the vehicle’s placard for inflation pressure. If you’re considering a size change, verify the complete setup before ordering; a tire that mounts on the wheel is not necessarily the right fit for the vehicle.
