You should always buy new tires when your current tires are worn out. Worn out tires do not give you the same performance that you used to get when your tires were in good condition. If you feel that the performance is declining, you should always check the tread depth to see if you are below 4mm depth. If the tread depth is above 4mm, there might be other issues that are lowering the performance of the tires. You should also check the tire pressure to make sure that you have the correct pressure in all the tires. Very low pressure can make the car skid and feel like you have a flat tire. Low pressure can also damage the sidewalls of the tires. It will also increase the heat build-up, which increases the tire wear and it will increase the rolling resistance, which translates to higher fuel consumption.
Over-inflated tires are also not good as it changes the profile of the tire and the contact area between the road and the tire. So you tend to have worse grip and not optimal performance. Braking distance will be prolonged, which increases the risk of accidents. Low tread depth will also prolong the braking distance and will increase the risk for aquaplaning. Aquaplaning needs a high groove volume to be able to disperse as much water as possible to ensure that the tire doesn’t lose contact with the road surface. Failure to get rid of the water will cause the tire to lose contact with the road. When the tire doesn’t have any contact, you will lose the ability to steer the vehicle. It is, of course, a situation that you want to avoid.
Modern tires have improved a lot in this area. New innovations in tread pattern and design to expel water more efficiently have been introduced, preventing the risk of aquaplaning. If you plan to purchase new tires, always look for tires that are good at preventing aquaplaning. If you need winter tires, then it is a different ball game, even though you can still have slush planing, where the wet snow will cause the same kind of effect as water in summer. Similar tire innovations have been introduced to prevent slush planing.
For winter tires you also want to ensure that you have the best possible grip for the conditions that you will be driving in. If you mainly will encounter icy roads, studded tires are your number one choice. They have a superior grip on ice and on tightly packed snow. If ice is rare and you more encounter snow, you can also consider non-studded tires. On snow, they perform basically equally, so it is often up to personal preference when the choice is made. If you select high-quality tires, both will provide you with a safe and comfortable driving experience.