If you decide to replace only two tires on your car, the new tires may have a deeper tread depth than the old tires that were not replaced. It is generally allowed to mix tires with different tread depths on non coaxial surfaces.  

The tire industry suggests that new tires should be installed on the rear axle.  

It is usually recommended to install high grip tires on the rear axle to prevent the vehicle from oversteering and loss of driving stability on wet and slippery roads. Some car manufacturers suggest installing the latest tires on the front axle. Consult car manufacturers and/or tire experts for more information.

Some vehicles may be equipped with tires of different specifications on the front and rear axles when they leave the factory.