The Rubber Manufacturer's Association (RMA) and every major tire manufacturer recommend mounting the new tires on the rear axle for your safety. Here's why:
The worn rear tires have less tread as a result less grip or traction on wet roads causing the rear end of the vehicle to fishtail or swing around out of control
The driver can steer and control the front of the vehicle to compensate for the worn tires on the front. Deeper tread on the rear axle will provide greater grip on wet roads and better overall vehicle control.
For more information on this subject visit www.TireSafety.com to watch their video.