You should always buy new tyres when your current tyres are worn out. The big question is maybe when are they worn out and how do I know it? When you go below 4 mm tread depth of your tyres, means that you will lose a lot of the safety properties associated with the tyres. These include grip, braking distance, ability to prevent aquaplaning and wet grip. These are key for keeping you safe. You will need to regularly check the tread depth of your tyres so that you are aware as you approach the 4 mm mark.
If you feel that you are not getting the same performance from your tyres than you used to get or feel that it is declining, then you should always check the tread depth to see if you are below 4mm depth. If the tread depth is above 4mm and you still have bad performance, there might be other issues that are lowering the performance of the tyres. You should then check the tyre pressure to make sure that you have the correct pressure in all the tyres. Very low pressure can make the car skid and feel like you have a flat tyre. Low pressure can also damage the sidewalls of the tyres, as well as increasing the heat buildup in the tyre. This increases the tyre wear and will increase the rolling resistance. High rolling resistance translates to higher fuel consumption.
Over inflated tyres is also not good as it changes the profile of the tyre, so you will change the contact area between the road and the tyre. This leads to worse grip and not optimal performance. Braking distance is prolonged, so that you might not be able to avoid accidents. Low tread depth will also prolong the braking distance and increase the chance for aquaplaning. Aquaplaning will need a high groove volume to disperse as much water as possible to ensure that the tyre doesn’t lose contact with the road surface. Failure to get rid of the water will cause the tyre to lose contact with the road. When the tyre doesn’t have any contact, you will lose the ability to steer the vehicle and an accident could be imminent.
Modern tyres have improved a lot in this area. New innovations in how to design the tread for optimal water removal have been implemented. It has significantly reduced the chance for aquaplaning. The main cause for accidents during thunderstorms is due to aquaplaning, so the danger should be realized so that if you plan to purchase new tyres, you need tyres that are good at preventing aquaplaning. If you need winter tyres, the they need to be able to prevent slushplaning, where the wet snow will cause the effect as with water. Similar innovations have been made here to prevent slushplaning.
For winter tyres you also want to ensure that you have the best possible grip for the conditions that you will be driving in. Often a good timing to change to new tires is when you are changing between winter and summer tyres.
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