Choosing replacement car tyres is not just a matter of matching the wheel diameter. This tyre size guide explains how to read sidewall markings, check load and speed ratings, confirm vehicle fitment, compare equivalent sizes safely, and use a practical checklist before you buy tyres online.
Overview
A tyre marking such as 225/45 R17 94W contains several separate pieces of information. Reading from left to right:
- 225 is the nominal section width in millimetres.
- 45 is the aspect ratio. The sidewall height is approximately 45% of the tyre’s width.
- R identifies radial construction, which is used by most modern car tyres.
- 17 is the wheel rim diameter in inches.
- 94 is the load index.
- W is the speed rating.
For example, 225/45 R17 tyres are about 225 mm wide, have a sidewall height of roughly 101 mm, and fit a 17-inch rim. A 205/55 R16 tyre is narrower, has a taller sidewall relative to its width, and fits a 16-inch rim. These dimensions are not interchangeable simply because both are common passenger-car sizes. The vehicle, wheel, clearance, load requirements, and approved alternatives must all be considered.
The most reliable starting point is your vehicle manufacturer’s information. Check the driver’s door-jamb label, fuel-flap label, handbook, or the vehicle’s official fitment information. Record the complete marking for each axle, including any different front and rear sizes. A tyre shop or online fitment tool can then confirm the result, but the vehicle’s own specification should remain the reference point.
Checklist by scenario
Replacing tyres with the same size
- Photograph or write down the complete sidewall marking, including load index, speed rating, and any XL, run-flat, or manufacturer-specific marking.
- Compare it with the vehicle label rather than relying only on the tyre currently fitted. A previous owner may have installed an unsuitable size.
- Check whether the vehicle uses different front and rear dimensions. This is known as a staggered setup, and it can limit tyre rotation. See the staggered tyre setup guide before ordering.
- Confirm that all replacement tyres have adequate load and speed ratings for the vehicle.
Considering an alternative size
Do not substitute a different size based on appearance or a calculator alone. A suitable alternative should preserve safe load capacity, provide adequate clearance from suspension and bodywork, work with the wheel width, and avoid unacceptable changes to rolling circumference. It should also be compatible with the vehicle’s electronic systems and any four-wheel-drive requirements.
Ask a qualified tyre professional to verify alternatives before purchase. A change in overall diameter can affect speedometer readings, gearing, ground clearance, braking behaviour, and the operation of systems such as traction or stability control.
Buying tyres online
- Enter the exact registration, vehicle details, or tyre specification into the retailer’s fitment tool.
- Compare the complete code, not just the width and rim diameter.
- Check whether the listing is for one tyre, a pair, or a full set.
- Confirm delivery, fitting, valve, balancing, and disposal arrangements before checkout.
- Inspect the tyres on delivery and raise any fitment or condition concern before they are fitted.
What to double-check
Load index
The load index is a code, not a weight in kilograms printed directly on the tyre. It represents the maximum load permitted for one tyre under specified conditions. Common examples include:
| Load index | Approximate maximum load per tyre |
|---|---|
| 91 | 615 kg |
| 94 | 670 kg |
| 95 | 690 kg |
| 97 | 730 kg |
| 100 | 800 kg |
| 104 | 900 kg |
| 109 | 1,030 kg |
This is only a selection from a larger tyre load rating chart. Use the manufacturer’s required rating, particularly for SUVs, 4x4s, vans, towing, and heavily loaded vehicles. A higher load index is not automatically a suitable upgrade if it changes the tyre’s construction, clearance, or required inflation pressure.
Speed rating
The speed rating is the final letter in the service description. Common ratings include T for up to 190 km/h, H for up to 210 km/h, V for up to 240 km/h, W for up to 270 km/h, and Y for up to 300 km/h under the relevant test conditions. These are capability classifications, not recommended driving speeds. Match or exceed the vehicle manufacturer’s specified rating unless a qualified specialist confirms an appropriate seasonal exception.
Special markings and pressure
XL or reinforced tyres may be required for certain vehicles and loads. Run-flat tyres need compatible vehicle equipment and may have specific replacement rules. Some cars also require tyres carrying a manufacturer-specific marking. Check these details before selecting a cheaper or apparently equivalent option.
Tyre pressure is separate from the number moulded into the sidewall. Use the vehicle’s recommended cold pressures from the door label or handbook, with the higher-load setting when instructed for passengers or luggage. For related maintenance guidance, read how long tyres last and review the advice on damage in the tyre sidewall damage guide.
Common mistakes
- Matching only the rim size: A 17-inch rim does not identify width, profile, load index, or speed rating.
- Assuming a larger tyre is better: Wider or lower-profile tyres can alter comfort, clearance, steering response, and wet-road behaviour.
- Ignoring axle differences: Some vehicles require different sizes front and rear or prohibit rotation between axles.
- Using the maximum sidewall pressure: That figure is not normally the vehicle’s recommended operating pressure.
- Mixing unsuitable tyre types: Keep seasonal, run-flat, and construction requirements consistent with the vehicle and axle.
- Buying by price alone: When comparing cheap tyres, also check wet grip, noise, load rating, speed rating, intended use, and the seller’s fitment information. The cheap tyre buying guide provides a useful comparison framework.
When to revisit
Return to this checklist whenever you change vehicle, wheels, tyre type, or driving requirements. Recheck the specification before seasonal changes if you are moving between summer, winter, or all season tyres. Revisit it before towing, carrying heavy loads, fitting a wheel-and-tyre package, or ordering tyres for a commercial van or SUV.
Before placing an order, complete these final actions:
- Read the vehicle label and record every approved tyre size.
- Match width, aspect ratio, construction, rim diameter, load index, and speed rating.
- Confirm special markings, axle position, and seasonal requirements.
- Ask for professional confirmation if changing size, wheel width, offset, or tyre construction.
- Save the final fitment details with your vehicle records for the next replacement.
If a replacement follows a puncture, sidewall bulge, or other damage, establish whether the tyre can safely be repaired or must be replaced before ordering; the puncture repair guide covers that decision. Correct identification is the simplest way to buy car tyres online with fewer delays and a better chance of getting the right fit first time.
