Driving on mismatched tires can cause premature and uneven wear on any new tires that are put on the vehicle. It can also cause the handling of the vehicle to be messed up because of the different wear and tread. Another expensive and dangerous thing can cause premature wearing on the braking system, drive train, and suspension. It may seem easier on the wallet to not buy all new tires but in the long run it will cost more, and most importantly, can be unsafe for you and others on the road.