Decoding Your Tyre Size — It's Simpler Than You Think
Every tyre has a code on its sidewall that tells you exactly what it is. Let's decode 205/55R16 91V:
- 205 — Tyre width in millimetres (the width of the tread that contacts the road)
- 55 — Aspect ratio (the sidewall height as a percentage of the width — so 55% of 205mm = 112.75mm sidewall)
- R — Radial construction (virtually all modern car tyres are radial)
- 16 — Rim diameter in inches (the wheel size this tyre fits)
- 91 — Load index (how much weight the tyre can support — 91 = 615kg per tyre)
- V — Speed rating (the maximum sustained speed — V = 149mph)
Where to Find Your Correct Tyre Size
Three easy places to check:
- Your current tyres: Look at the sidewall of any existing tyre. The size code is moulded into the rubber. (But only if you're sure the current tyres are correct — if someone fitted the wrong size before, you'd be copying their mistake.)
- Driver's door jamb sticker: Most vehicles have a tyre information sticker on the driver's door frame showing the recommended sizes and pressures.
- Owner's manual: Lists the original equipment tyre size and any approved alternatives.
Pro tip: If you can't find it, just tell TyreBlitz your vehicle make, model, year, and engine size when you call. We'll look it up in seconds.
What Happens If You Fit the Wrong Size
Getting the tyre size wrong isn't just a technicality — it has real consequences:
Wrong Width (e.g., 225 instead of 205)
A wider tyre may rub against the wheel arch during cornering or over bumps, causing damage to the tyre and bodywork. A narrower tyre reduces grip and changes the handling balance. Either way, it affects your speedometer accuracy and can void your insurance.
Wrong Aspect Ratio (e.g., 65 instead of 55)
A higher profile (taller sidewall) increases the overall wheel diameter, which throws off your speedometer, ABS, traction control, and stability systems. The car may read 30mph when you're actually doing 33mph. Over a 60mph journey, that error accumulates.
Wrong Rim Diameter (e.g., 17 instead of 16)
This is the most obvious mistake — the tyre simply won't fit on the wrong size wheel. If someone's sold you a 17-inch tyre for a 16-inch wheel, send it back.
Wrong Load Index or Speed Rating
Fitting a tyre with a lower load index than specified means the tyre may not be able to support the weight of your vehicle safely — especially when fully loaded. A lower speed rating means the tyre isn't designed for the speeds your vehicle is capable of. Both are MOT failures and insurance issues.
Can I Fit a Different Size to My Car?
Sometimes, yes — but only within approved parameters. Many vehicles have multiple approved tyre sizes (listed in the owner's manual or on the door jamb). For example, a car might be approved for both 205/55R16 and 225/45R17. Both are correct — just different wheel sizes with different profile tyres that result in approximately the same overall diameter.
Golden rules:
- All four tyres should be the same size (unless your vehicle is designed for staggered fitment — e.g., wider rears on some sports cars)
- On the same axle, tyres must always be the same size, brand, type, and tread pattern
- Never mix radial and cross-ply tyres (extremely rare now, but technically still worth noting)
- Load index and speed rating must meet or exceed the manufacturer's specification
Not Sure About Your Tyre Size? Call Us.
It takes us about 10 seconds to look up the correct tyre size for your vehicle. Just call 0208 079 1031 with your reg plate or vehicle details, and we'll tell you exactly what you need. We'll also tell you what's available on our van right now if you need a same-day fitting.