255/45 R21 Tyres

The numbers 255/45 R21 are 3 very important pieces of information in regards to a tyre's size. They tell you the width, profile and diameter of a tyre. For a tyre that is 255/45 R21 this information can be broken down into the following: 255 - the width in millimetres of the tyre. In this case 255mm wide. 45 - the height of the sidewall as a percentage of the width. For this tyre that is 45% R21 - the diameter, in inches, of the tyre's inner rim. Letting you know what size of wheel this tyre should be fitted to. For this size, that is a 21 inch wheel

There is no standard recommended tyre pressure for 255/45 R21 tyres, as each vehicle manufacturer will recommend their own optimal tyre pressure so you can enjoy the best performance from your vehicle. If you are unsure as to the correct tyre pressure for your 255/45 R21 tyres, there are a few different places you can look to find this information. Vehicle handbook - There is a section in most manufacturer handbooks which covers all the technical details for a tyre. This includes the tyre size, load rating, speed rating and tyre pressure. Tyre label - The tyre label is often found on the door jamb of the driver’s side. It is a small rectangular sticker which contains the information you need to know in regards to tyre pressure and tyre sizes. It should also tell you the tyre pressure recommendation

The height of your tyre's sidewall for the tyre size 255/45 R21 can be worked out by using the tyre's profile size. For the tyre size, the profile is 45. What this tells us is that the sidewall height is 45% of the cross-section width of the tyre. The sidewall height is determined by the distance between the rim line and the tread line. So, as this tyre has a cross-section width of 255mm, the height of the sidewall is 45% of 255 – which is 114.75mm.

Yes, it is very important that the tyre size of a tyre is exact. As the only part of a vehicle in contact with the road, there is a lot of reliance on tyres to be accurate, correctly constructed and exact. Different parts of the vehicle require accurate tyres so they can function correctly. The speedometer, for example, is only able to accurately calculate the speed and distance you are travelling if the tyres are exact and what it has been calibrated to. This is also true for the different section of your vehicle which deal with the handling, braking and suspension of your vehicle. These all need accurate and exact tyres as they have been calibrate to function based on tyres of a specific size. If the tyres were not exact your vehicle’s performance and the your safety would suffer.

There is no standardised recommendation on whether tyres should be the same on the front and back, as this depends on the vehicle you drive and the requirements as set out by the vehicle manufacturer. As a general rule of thumb, you will find that front-wheel drive vehicles tend to have the same size on all axles, whilst rear-wheel drive vehicles tend to require wider wheels and tyres on the rear of the vehicle. Please make sure that you check the requirements of your specific vehicle. Each model of vehicle is different and requires different criteria of it’s tyres to ensure the best performance. Check your vehicle handbook for the right tyre size(s) for your vehicle.