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Frequently Asked Questions About Tyres
What does 305/30 R21 mean?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 305/30 R21 this is broken down as follows: 305 - the width of the tyre in millimetres. In this case 305mm 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
If you are looking for the recommended tyre pressure for 305/30 R21 tyres, you will find that this will depend on the manufacturer and model. Each vehicle manufacturer has their own tyre pressure recommendation for their model, that will deliver an optimal performance for your vehicle. Check the following if you are unsure what the correct tyre pressure is for your 305/30 R21 tyres Tyre label - this is often the best place to find your tyre pressure information. It is located on the door jamb on the driver’s side of the vehicle. It is a small rectangular label that provides the driver with the recommended tyre pressure and tyre sizes. Vehicle handbook - Quite often the vehicle handbook with contain a section that relates to the technical details for the tyre. This should include the required tyre size, load rating, speed rating and recommended tyre pressure.
The sidewall height of a 305/30 R21 tyre is calculated by using the profile information of the tyre size. In this instance 30. This tells us that it is 30% of the tyre's cross-section width. When looking at a tyre, the sidewall height is the distance between the tread line of the tyre and the rim line. Given that this tyre size has a cross-section width which equals 305mm, we can work out that the sidewall height is 30% of 305 – this comes to 91.5mm.
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.