Tyre sizes – everything you need to know to choose the right one
Thinking about changing your tyres and wondering which size you need? We're here to help!
Understanding tyre sizes is essential for making the right choice and ensuring your vehicle runs smoothly. Whether you're replacing your current tyres or considering a size change to improve performance or aesthetics, we'll explain everything you need to know about tyre dimensions.
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What are the most popular tyre sizes?
Tyrecomp.ie shows you below the most popular sizes chosen by our users over the past 12 months. Click on any link to see the best prices and all the key information for tyres in that size.
Top tyre sizes for cars and SUVs
Top 4x4 tyre sizes
Top motorbike tyre sizes
Top van tyre sizes
Top lorry tyre sizes
How to read a tyre size
European tyre sizes (Metric or Hard Metric)
The tyre size is marked on the sidewall and usually looks like this:
- 205: Tyre width in millimetres.
- 55: Sidewall height (aspect ratio as a percentage of width).
- R: Type of construction (Radial).
- 16: Inner diameter (wheel size) in inches.
- 91: Load index (maximum weight the tyre can carry).
- V: Speed rating (maximum speed the tyre can safely handle).
American tyre sizes
P-Metric and LT-Metric
These two systems use the same format as the standard metric system but are prefixed with P for 'Passenger' or LT for 'Light Truck'.
Examples: P215/65 R15 or LT245/75 R16
4x4 and off-road tyre sizes - Flotation Tyres
Used mainly for off-road tyres and some 4x4s, the Flotation system expresses dimensions in inches without using an aspect ratio.
Example: 31x10.50 R15
- 31: Overall tyre diameter in inches.
- 10.50: Tyre width in inches.
- R15: Wheel diameter in inches.
Where to find your tyre size
To find the correct tyre size for your vehicle, you can:
- Check the vehicle manual: It lists standard tyres and optional fitments.
- Check the sticker on the driver’s door pillar: It shows approved tyre sizes and recommended pressures.
- Look inside the fuel flap: Some manufacturers display tyre size information there.
- Check the sidewall of your current tyres: The size is always marked on the side.
For a complete explanation, check out our page « How to read a tyre? ».
Everything you need to know about changing tyre size
Why change tyre size?
Grip
Aesthetics
Cost
Adaptation
There are many reasons why you might want to change your tyre size:
- Performance improvements: Better grip and more responsive handling.
- Aesthetics: Give your vehicle a sportier or more customised look.
- Availability and price: Find alternative sizes that are easier to source or cheaper.
- Adaptation to driving conditions: Prepare your vehicle for specific use cases (off-road, snow, etc.).
Changing tyre size isn’t a trivial decision. It will impact your vehicle, so you need to weigh up the pros and cons carefully. (This usually involves fitting larger wheels and lower-profile tyres.)
Main advantages of changing tyre size
- Better dry grip: A wider contact patch improves grip, especially when cornering.
- More responsive steering: Low-profile tyres offer quicker, more precise handling.
- Improved aesthetics: Larger wheels and tyres give your vehicle a more dynamic look.
- Better braking performance: A larger contact area improves braking, especially on dry roads.
- Increased stability at high speeds: Wider tyres offer better high-speed stability.
- Specific adaptations: Suitable for sporty driving or off-road use.
Main disadvantages of changing tyre size
- Higher costs: Larger tyres and wheels are generally more expensive.
- Higher risk of aquaplaning: Wide tyres evacuate water less effectively.
- Increased fuel consumption: Higher rolling resistance and heavier wheels can raise fuel usage.
- Reduced comfort: Low-profile tyres absorb bumps and road imperfections less well.
- More road noise: Wide tyres are often louder than standard ones.
- Reduced snow performance: Wide tyres generally perform worse on snow-covered roads.
- Changes in vehicle behaviour: Tyre size changes can affect speedometer accuracy and overall driving dynamics.
Tyre size changes: NCT and insurance
To pass the Irish NCT (National Car Test) your tyres must either: ① match a dimension shown on the vehicle’s Certificate of Conformity (CoC) / manufacturer plate, or ② be another size that the vehicle-maker approves and that is compatible with the ETRTO data tables. They must keep the same construction type (radial/diagonal) and carry at least the original load and speed index. If a wheel- or tyre-change affects safety-critical systems or body clearance, an Engineer’s Report may be required and the alteration must be entered on the Vehicle Registration Certificate (Form RF111 – ‘Change of Particulars’).
With insurance in Ireland, any un-notified tyre-size change can be treated as a ‘material change’ under Section 8 of the Consumer Insurance Contracts Act 2019 (as amended 2022). Always inform your insurer before fitting a non-standard size, otherwise cover may be reduced or void in the event of a claim.
Manufacturer or ETRTO alternatives
Irish test centres use the ETRTO data tables to confirm that an alternative tyre fits the approved rim width and load/speed requirements. Consult these tables—or obtain written confirmation from the tyre manufacturer—before purchasing. The workshop that fits the tyres is responsible for roadworthiness and may recommend a voluntary pre-NCT inspection if in doubt.
Can I fit narrower winter tyres?
Yes. Winter or all-season tyres marked 3PMSF may carry a lower speed symbol (minimum ‘Q’) and can be slightly narrower, provided they still meet or exceed the original load index and are approved for the rim width. If the speed rating is lower than the original summer specification, an in-car sticker showing the reduced maximum speed must be placed in the driver’s field of view (RSA Tyre Safety Guide, 2019).
Example of a tyre size change
Changing the tyres
Here’s an example comparing two tyre sizes: switching from 195/65 R15 to 205/55 R16.
Tyre width:
- 205 mm: for 205/55 R16.
- 195 mm: for 195/65 R15.
Sidewall height (aspect ratio):
- 55% of 205 mm for 205/55 R16, i.e., 112.75 mm.
- 65% of 195 mm for 195/65 R15, i.e., 126.75 mm.
Wheel diameter:
- 16 inches for 205/55 R16, i.e., 406.4 mm (16 × 25.4 mm).
- 15 inches for 195/65 R15, i.e., 381 mm (15 × 25.4 mm).
Overall tyre diameter:
- 631.9 mm for 205/55 R16 (2 × 112.75 mm + 406.4 mm).
- 634.5 mm for 195/65 R15 (2 × 126.75 mm + 381 mm).
Impact on the vehicle
- Handling: Wider, lower-profile tyres improve grip and responsiveness.
- Comfort: Taller sidewalls (195/65 R15) offer better comfort.
- Fuel consumption: Wider tyres may slightly increase fuel use.
- Aesthetics: 16-inch wheels give a sportier look.
Comfort
Grip and responsiveness
Fuel efficiency
Sporty look
