245 is the tyre width in millimetres, 40 is the aspect ratio (the sidewall height is 40% of 245 mm, so about 98 mm), R means radial construction, and 20 is the wheel diameter in inches. Choosing the correct size helps maintain the intended handling, braking performance and speedometer accuracy.
Only if the alternative size is approved for your exact vehicle and wheel. 245/45 R20 is noticeably taller in overall diameter, while 255/35 R20 is shorter, and either change can affect clearance, gearing and speedometer readings. Check the tyre size options on your door jamb sticker or handbook, and if you are unsure, stick to the listed sizes to stay compliant and avoid fitment issues.
Use the load index and speed rating shown on the vehicle placard (usually on the door frame) or in the handbook, as the correct rating can vary by model, engine and axle weight. Many tyres in this size are offered with higher ratings such as W or Y and may also be available as XL (extra load). When ordering, match the required rating exactly, and avoid mixing different load or speed ratings across the same axle unless the vehicle manufacturer explicitly allows it.
Choose XL only if your vehicle requires a reinforced tyre, which is common on heavier or more powerful cars using this size, and will be stated on the placard or in the handbook. Run-flat should be selected only if your car is designed for it and you want the ability to drive a short distance after a puncture, as ride comfort can be firmer. If your vehicle was not supplied with run-flats, confirm suitability before switching.
Summer tyres suit most UK conditions and offer strong grip and steering feel in mild to warm temperatures. If you regularly drive in colder weather or expect snow and slush, winter tyres improve traction and braking in low temperatures. All-season tyres are a practical compromise for year-round use when you want better cold-weather capability without changing tyres twice a year, so choose based on where you drive and the temperatures you see most often.
