Knowing how to read a tyre size is only the first step. This guide explains every number and letter on the sidewall, provides practical load and speed rating charts, shows why apparently similar sizes are not automatically interchangeable, and gives you a reusable checklist for confirming tyre and wheel fitment before you buy.
Overview
A marking such as 225/45 R17 94W contains several separate measurements and ratings:
- 225: section width in millimetres, measured at the tyre’s widest point.
- 45: aspect ratio. The sidewall height is approximately 45% of the 225 mm width.
- R: radial construction, used by most modern car tyres.
- 17: wheel rim diameter in inches. A 17-inch tyre must be fitted to a 17-inch rim.
- 94: load index, indicating the maximum load assigned to one tyre under the relevant rating conditions.
- W: speed symbol, indicating the tyre’s certified speed category under specified test conditions.
You may also see markings such as XL or Reinforced for increased load construction, Run Flat branding, or vehicle-specific original-equipment markings. Treat these as part of the specification, not optional decoration. Your vehicle handbook, door-jamb placard or fuel-flap label should be the starting point when you buy car tyres online or compare tyres near you.
Quick tyre-size decoding example
| Marking | Meaning | Fitment implication |
|---|---|---|
| 205/55 R16 | 205 mm wide, 55% aspect ratio, radial, 16-inch rim | Requires a compatible 16-inch wheel and approved vehicle clearance |
| 225/45 R17 | 225 mm wide, 45% aspect ratio, radial, 17-inch rim | May be a factory or optional size on some cars, but never assume compatibility |
| 91V | Load index 91 and V speed symbol | Both ratings must meet the vehicle requirement |
Tyre dimensions can be compared mathematically, but a similar rolling diameter does not prove that a tyre fits. Width, rim width, suspension clearance, bodywork clearance, gearing, steering angle and electronic systems can all matter.
Checklist by scenario
Replacing tyres with the same specification
- Read the complete sidewall marking, including load index, speed symbol and any special construction marking.
- Compare it with the vehicle placard and handbook. If the car has different front and rear sizes, record each axle separately.
- Check whether the replacement tyre is suitable for the intended use: summer, winter, all-season, SUV, 4x4, van or run-flat application.
- Confirm that all four tyres meet the required specification. Do not mix incompatible construction types or use a lower load or speed rating without professional confirmation.
- Check the tyre’s condition, date code and storage history when buying older stock or used tyres.
Considering an alternative tyre size
- Calculate the approximate overall diameter: rim diameter plus twice the sidewall height. Sidewall height equals width multiplied by aspect ratio.
- Compare the proposed size with an approved option listed by the vehicle manufacturer or a reputable fitment system.
- Check that the replacement rim width is within the tyre maker’s permitted range.
- Confirm speedometer, traction-control, anti-lock braking and driver-assistance implications with a qualified specialist.
- Check full-lock steering and suspension clearance, not just the gap while the vehicle is parked.
Buying a wheel and tyre package
- Confirm the wheel’s diameter, width, PCD, centre bore and offset.
- Match the wheel’s load capacity to the vehicle and axle loads.
- Check whether the wheel needs hub-centric rings, different wheel bolts or nuts, or TPMS-compatible components.
- Verify that the tyre size is approved for the wheel and does not contact brake calipers, suspension parts or bodywork.
- Ask for the complete specification in writing before ordering an alloy wheel package.
Common size comparisons
The following are mathematical examples, not universal fitment approvals. They show why people often compare certain sizes, but a vehicle-specific check remains essential.
| Reference size | Often compared with | Approximate diameter relationship | Important limitation |
|---|---|---|---|
| 205/55 R16 | 225/45 R17 | Very close overall diameter | Width, rim width, offset and clearance may differ |
| 195/65 R15 | 205/55 R16 | Very close overall diameter | Only valid where the vehicle approves both sizes |
| 215/60 R16 | 225/55 R17 | Similar diameter range | Check load rating, wheel width and physical clearance |
What to double-check
Load rating chart
Load index is a coded value rather than a kilogram measurement printed directly on the sidewall. The entries below are commonly encountered examples; use the vehicle’s required rating and the tyre manufacturer’s technical information for a buying decision.
| Load index | Maximum load per tyre | Load index | Maximum load per tyre |
|---|---|---|---|
| 88 | 560 kg | 94 | 670 kg |
| 89 | 580 kg | 95 | 690 kg |
| 90 | 600 kg | 96 | 710 kg |
| 91 | 615 kg | 97 | 730 kg |
| 92 | 630 kg | 98 | 750 kg |
| 93 | 650 kg | 99 | 775 kg |
Do not select a lower load index because the tyre physically fits. Heavy loads, towing, commercial use and larger vehicles may require substantially higher ratings. Van tyres and some SUV or 4x4 tyres can have construction and load requirements that differ from ordinary passenger-car tyres.
Speed rating chart
| Symbol | Certified category | Symbol | Certified category |
|---|---|---|---|
| T | Up to 190 km/h | V | Up to 240 km/h |
| H | Up to 210 km/h | W | Up to 270 km/h |
| Y | Up to 300 km/h | (Y) | Above 300 km/h under the applicable designation |
Speed symbols describe a test category, not a recommended driving speed. Winter tyres may use a different approved speed rating, and vehicle guidance may specify conditions for that choice. Always follow the handbook, tyre placard and applicable local requirements.
Wheel measurements
PCD means pitch circle diameter: the diameter of the circle passing through the centres of the wheel’s bolt holes. A wheel with the wrong PCD is not made safe by tightening the fasteners. Offset is the distance between the wheel’s mounting face and its centreline. Excessive change can affect inner clearance, outer bodywork clearance, steering feel and bearing loads. Centre bore is the hole locating the wheel on the hub; a larger bore may require correctly specified hub-centric rings, while a smaller bore will not fit over the hub.
Common mistakes
- Matching only the first number: Two tyres can share a width but differ in diameter, load capacity or speed rating.
- Assuming a close diameter means a safe fit: Wheel width, offset and brake clearance can still make the combination unsuitable.
- Ignoring axle differences: Some vehicles use different front and rear sizes or directional fitments.
- Using the maximum sidewall pressure: The pressure moulded on the tyre is not necessarily the vehicle’s recommended cold pressure. Use the placard or handbook; see the tyre pressure guide for checking steps.
- Mixing seasonal sets without checking: Review size, load, speed rating, rotation direction and storage condition before fitting winter or all-season tyres.
- Forgetting accessories: New wheels may need compatible fasteners, valves, sensors or locking nuts.
- Relying on a generic online listing: A product labelled “fits” should still be checked against the exact vehicle, engine, axle and trim.
For a broader explanation of PCD, offset, centre bore and clearance, use the wheel and tyre fitment checker guide. If the purchase is for a long trip, pair this check with the pre-trip tyre safety checklist.
When to revisit
Return to this checklist before each seasonal change, whenever you move to a new wheel size, and before ordering a wheel and tyre package. Recheck the vehicle placard if wheels, suspension or brakes have been modified. For an existing set, inspect pressure, tread, damage and age regularly; the tyre age and date-code guide explains what to look for.
This guide should be reviewed when load-index or speed-rating definitions, labelling rules or technical standards change. Its size-equivalence examples and vehicle-fitment examples should be refreshed annually and whenever major tyre ranges or original-equipment fitments are revised. New commonly searched sizes can be added quarterly based on purchasing patterns. Before you act, write down the complete current marking, compare it with the vehicle documentation, confirm wheel measurements, and obtain a vehicle-specific clearance check if anything differs from the original specification.
