It's always best to replace all four of course, reason being is for traction. If you get caught in the rain and you have to hit the brakes, the tires in the back will definitely help out if you just replaced those, but say your going around a corner, and you replaced your rears, but not the fronts, depending on your speed and the tread depth of your fronts, you could easily just keep going forward which could cause you to crash into something. Not only that the legal tread depth is 3/32 in which you have to replace them. There are a lot of factors to why its best to replace all four, giving its necessary, so feel free to contact me with any tire and wheel questions. I work at Discount Tire / America's tire.

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2002 Ford Ranger Thunderbolt

V6 3.0L

Body kit
Duck tail spoiler
Non-Functional hood scoop
Carbon fiber interior
Cold-air intake box w/ K&N cone filter
BOSCH sparks and wires
MB wheels

Still more to come...