Originally Posted by spearfish25

You should base your decision of changing one or all tires upon tread depth and not mileage. The way I drive, my tires are nearly bald by 8k miles. If your tread depth is below about half of new (new is usually 9 or 10/32"...minimum legal is 2/32"), then you should change all the tires. Otherwise your wet performance on the older tires will be impaired compared to the one new tire leading to imbalance in grip and a potentially dangerous situation.