The reason you are buying new car tires sooner than recommended is perhaps because you are allowing the tread on the tires to wear too quickly. Pay closer attention to the condition of the tires and you can stretch out the life by months or years!
The worn areas of the tire might appear to be nothing at first look, but they are affecting what happens when you drive. Not only will the tires need to be replaced sooner, the car is having trouble being able to grip the highway at high speeds or bad weather.
