Reading a tyre size correctly is the first step toward buying compatible car tyres. This guide explains what each number and letter means, then provides a reusable tyre fitment checklist for replacements, seasonal changes, wheel upgrades and online orders.
Overview
A tyre sidewall contains more information than just the tyre’s width. A common example is 225/55 R17 97V. Each part identifies a specification that affects compatibility, handling, load capacity or approved operating speed:
- 225 is the nominal tyre width in millimetres, measured across the tyre from sidewall to sidewall.
- 55 is the aspect ratio. The sidewall height is approximately 55% of the tyre’s width. It is a proportion, not a measurement in millimetres.
- R indicates radial construction, which is used on most modern passenger vehicles.
- 17 is the wheel diameter in inches. A tyre marked R17 is designed for a 17-inch wheel, not a 16- or 18-inch wheel.
- 97 is the load index. This code corresponds to a maximum load capacity in the tyre manufacturer’s load-index table.
- V is the speed rating. It identifies the tyre’s rated speed category under specified conditions; it is not a recommendation to drive at that speed.
You may also see extra markings such as XL for extra-load construction, seasonal symbols, run-flat designations, manufacturer-specific approvals or electric-vehicle suitability markings. These details can matter when comparing otherwise similar tyres.
The size printed on the sidewall is useful, but it should not be the only fitment check. The vehicle manufacturer’s placard, owner’s manual and a reliable vehicle-specific fitment listing should be treated as the reference points for the correct vehicle tyre size. If the car has more than one approved size, confirm which size applies to your wheel and trim level.
For more background, see this tire size and wheel fitment guide. It is also helpful to distinguish tyre size from wheel fitment: tyres must match the wheel diameter and suitable rim width, while the complete wheel must clear the brakes, suspension and bodywork.
Checklist by scenario
Replacing worn tyres with the same specification
- Read the full marking on the current tyre, including width, aspect ratio, construction, wheel diameter, load index and speed rating.
- Check the vehicle tyre placard, usually located on a door frame, fuel flap or another manufacturer-specified position.
- Confirm whether the vehicle requires a particular load rating, run-flat design or manufacturer approval.
- Order the correct quantity. If replacing only two tyres, ask a qualified tyre professional where they should be fitted based on the vehicle and tread condition.
- Check that the replacement tyres are suitable for the vehicle’s axle and intended use rather than relying on a size match alone.
Changing between summer and winter tyres
Start with the vehicle’s approved seasonal sizes rather than assuming the same size is suitable for every season. Some vehicles permit a narrower winter setup, while others require the same size front and rear. Confirm the approved combination before buying.
- Verify the load index and speed rating for the seasonal set.
- Check whether the wheels include compatible tyre-pressure monitoring sensors.
- Label stored wheel-and-tyre assemblies by position if they are directional or have different front and rear sizes.
- Inspect tread, age, cracks and sidewall condition before refitting.
- Use the manufacturer’s recommended cold inflation pressures, not the maximum pressure printed on the sidewall.
If you keep a second set, review how to store seasonal tyres properly so the tyres remain protected between seasons. The choice between winter tyres and all-season tyres also depends on local temperatures, road conditions and driving patterns.
Buying tyres online
When shopping for tyres online, enter the vehicle’s make, model, year and trim where the retailer provides a vehicle selector. Then compare the result with the markings on the vehicle and current tyres. Vehicle parts by make and model tools are helpful, but they can return more than one approved configuration.
Before adding a tyre to your order, check the complete description. Look for the exact size, load index, speed rating, construction type and any required special marking. Do not select a tyre solely because its width and wheel diameter look correct. If the listing and vehicle placard disagree, pause the order and confirm the fitment with the vehicle manufacturer, tyre maker or qualified installer.
Changing wheels or tyre sizes
A larger wheel usually requires a lower-profile tyre to keep the overall rolling diameter reasonably close, but the change is not automatically safe or legal in every application. Check rim width, offset, bolt pattern, centre bore, brake clearance, suspension clearance and fender clearance. Confirm that the resulting load capacity and speed rating meet the vehicle’s requirements.
Wheel spacers, unusual offsets and oversized combinations deserve additional care. Read the wheel spacer guide before treating spacers as a simple solution to a fitment problem.
What to double-check
Front and rear specifications: Some performance, luxury and four-wheel-drive vehicles use different front and rear sizes. Check all four positions instead of copying one sidewall marking across the vehicle.
Load index: A tyre that physically fits the wheel may still be unsuitable if its load index is below the vehicle requirement. This is especially important for SUVs, vans, vehicles carrying heavy loads and electric vehicles, which may have specific load requirements.
Speed rating: Match the vehicle requirement and intended use. A different rating should not be selected casually, particularly when the vehicle manufacturer specifies a minimum.
Overall diameter: Changes in diameter can affect ground clearance, gearing, speedometer readings, electronic driver aids and the available space around the wheel. Ask for a complete compatibility check before changing size.
TPMS compatibility: A tyre-pressure monitoring warning after a wheel change may relate to sensor compatibility, sensor damage or a required relearn procedure. Use this TPMS warning light guide when diagnosing the warning rather than assuming the tyre itself is defective.
Condition after fitting: Inspect for uneven seating, visible damage and valve problems. Have the wheels balanced and check alignment if the vehicle pulls, the steering wheel is off-centre or the tyres wear unevenly. Alignment and balancing address different issues, as explained in this wheel alignment versus wheel balancing guide.
Common mistakes
- Using only the first three numbers: 225/55 R17 is incomplete without the load index and speed rating.
- Confusing wheel diameter with tyre diameter: The number after R refers to the wheel diameter in inches.
- Assuming every tyre of the same size is identical: Tread pattern, construction, load rating, seasonal design and intended vehicle use can differ.
- Mixing unsuitable tyres: Combining different types or specifications can affect handling. Follow the vehicle maker’s guidance and seek professional advice when replacing only part of a set.
- Ignoring sidewall damage: A puncture repair is not appropriate for every location or type of damage. Review the tyre sidewall damage guide before driving on a damaged tyre.
- Assuming a cheaper tyre is equivalent: Compare the full specification and intended use, not just the headline price. A budget option may be suitable for one vehicle and unsuitable for another.
- Forgetting the spare: A temporary spare, full-size spare or repair kit may have separate instructions and limitations. Check it when reviewing the vehicle’s tyre equipment.
When to revisit
Return to this checklist before every tyre purchase, especially when the vehicle changes hands, wheels are replaced, a new trim level is fitted, or you move to a different climate. Recheck the vehicle placard whenever you are planning a seasonal change, towing, carrying heavier loads or switching to a different wheel size.
Before ordering, write down the approved front and rear sizes, minimum load indexes, required speed ratings and any special markings. Photograph the placard and current sidewalls if useful. Then compare those details with the complete online listing and ask an installer to confirm clearance and sensor requirements before fitting.
After installation, set the cold pressures, complete any required TPMS relearn, check for warning lights and monitor the first few miles for vibration, pulling or rubbing. If anything feels abnormal, stop and have the fitment inspected. Keeping this process with your maintenance records makes the next tyre change faster without turning an old assumption into an unsafe purchase.
