In a perfect world, all four tires would wear evenly so we could change them at the same time. However, even the best of us forget to rotate our tires and end up with two worn out tires and two with great tread depth. (You can check your tread depth accurately with an Accutire tread depth gauge).
Many ask in this case: "Do the new tires go up front or in the rear?".
If you have a front wheel drive car, conventional “old school” wisdom may suggest the new tires should be on the front. However, the combination of worn tires on the rear may cause some very unpredictable handling characteristics in the wet or snow, which can lead to an over-steer condition. Over-steer looks like this :
So, always install new tires on the rear of the vehicle. This is true for both front-wheel drive and rear-wheel drive vehicles.
For more information, read "Where to Install New Pairs of Tires?"