It means a 325 mm section width, a 30% profile, and a 21-inch rim diameter. It must match the size on your current tyre sidewall or your vehicle placard exactly, including any extra markings such as XL, run-flat or OE codes. If your wheels are aftermarket, confirm the rim width is within the tyre manufacturer’s approved range for 325/30 R21.
Use the values on your door-jamb placard or handbook as the minimum. This wide, low-profile size is often specified with higher load indexes and high speed ratings on performance vehicles, so it is important not to downgrade. Choose the same or higher rating than specified and avoid mixing ratings across an axle.
Most vehicles that use this size run it on the rear axle as part of a staggered setup, paired with a narrower front size. For consistent handling, replace tyres in axle pairs and match tread pattern and seasonal type left to right. If your car runs the same size all round, keep four identical tyres to maintain balance and traction.
Run-flat and XL (reinforced) options exist in 325/30 R21, but availability depends on the brand and tyre pattern. If your vehicle was supplied with run-flats, it is usually best to stay with run-flat unless the manufacturer allows a change and you accept different ride characteristics. If you need extra support, filter for XL and confirm it meets your required load index.
Summer tyres are the most common choice in 325/30 R21 and suit UK performance driving when temperatures are generally above 7C. Winter tyres are better for frequent cold, slush or snow, while all-season tyres target year-round use without seasonal changes. Winter and all-season options can be more limited in this width, so compare suitable brands early and select the category that matches your climate and mileage.
