315/30 R21 tyres

The first 3 sets of numbers that you will see when reading a tyre size tell you the width, profile and diameter of your tyre. In this case, for the tyre size 315/30 R21 this is broken down as follows: 315 - the width of the tyre in millimetres. In this case 315mm wide. 30 - the height of the sidewall as a percentage of the width. In this case 30% R21 - the diameter of the tyre's inner rim in inches. Essentially what size of wheel this is intended to be fitted onto. For this size, that is a 21 inch wheel

There is no standard recommended tyre pressure for 315/30 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 315/30 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 315/30 R21 can be worked out by using the tyre's profile size. For the tyre size, the profile is 30. What this tells us is that the sidewall height is 30% 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 315mm, the height of the sidewall is 30% of 315 – which is 94.5mm.

Yes, tyre sizes do need to be exact. There is a lot of calibration on a vehicle which relies on the tyre size. For example, the speedometer on your vehicle uses the diameter of your wheels and tyres to correctly work out the speed at which you are travelling. If the size was not exact, then the speedometer would not be able to deliver an accurate reading. Similarly, a tyre size that was not accurate would result in poor steering and grip from the car. As the wheels would be out of balance and the vehicle unable to perform to an optimal level. All of which could lead to issues with your safety and the performance of your vehicle.

This will depend on the vehicle you are driving. In the majority of cases you will find that front wheel vehicles have the same tyre size recommendations on the front and the back. However, there are instances when the suspension has been calibrated to perform with differing tyre sizes. It can be the case that rear wheel vehicles recommend wider tyres on the rear of the vehicle to increase the traction provided. Make sure that you always follow the manufacturer’s instructions and recommendations on tyre size for your specific vehicle. Check your vehicle handbook if you unsure as to what tyre sizes you require.