255 is the tyre width in millimetres, 35 is the sidewall height as a percentage of the width (a low-profile sidewall), and R21 means it fits a 21-inch wheel. Because the sidewall is short, this size tends to feel more direct in corners but can be firmer over potholes, so it is worth prioritising strong construction and the correct load rating.
Many vehicles that use 255/35 R21, especially performance SUVs and EVs, require XL (extra load) to meet the manufacturer load index. Check your door placard or handbook, then match the exact load index and speed rating when buying, as fitting a lower-rated tyre can affect safety, insurance and MOT compliance.
The physical size is the same, but ZR indicates the tyre is designed for higher speeds and it will carry a specific speed rating letter (such as Y or W). When replacing 255/35 R21 tyres, match or exceed the speed rating specified for your car, and keep the same rating across an axle.
Yes, 255/35 R21 is often used on the front with a wider rear tyre in a staggered setup. Replace tyres in pairs across the same axle, and aim to keep the same brand, model and tread pattern as the other axle where possible to maintain balanced grip and predictable handling.
There is no single correct pressure because it depends on vehicle weight, load and the exact tyre specification. Use the cold tyre pressures on the door jamb placard (often showing different values for normal and fully loaded driving), check pressures regularly, and recalibrate TPMS if your vehicle requires it after fitting new tyres.
