Since tires affect the personality and performance of your vehicle, all four tires should be as identical as possible or you may have handling problems. If your tires don't match, it is possible that one end of your vehicle won't respond as quickly or completely as the other, making it more difficult to control.
If you tires have a lot of remaining tread depth, but you need to replace just one that has been damaged by an accident, road hazard or a vandal, you should replace it with a tire that exactly matches the others. Select a replacement tire of the same brand, line, size and speed rating.
If two of your tires have a lot of remaining tread depth, but you need to replace the other two because they were damaged or have worn out, you should replace them with a pair of tires that come as close as possible to matching your existing tires. While identical new tires are desirable, others of the same size and type can also provide good results. Only consider selecting new tires that are from the same tire category as your existing tires. New tires should be installed on the rear axle.
If all of your tires are wearing out together, you have the greatest flexibility in tire selection. If you were happy with the original tires, simply replace them. If you want longer tread wear, a smoother ride or more handling, there are probably tires that will help you accomplish that.
Resources:
WWW.SEARCHWARP.COM
WWW.ENJOYTHEDRIVE.COM
WWW.BUZZLE.COM