Replace Old Back Tire Where to Replace New

If this is a two wheel drive car, you should replace the tires as a pair on the same axle.

If you loose grip on the rear of the car, it is more difficult to regain control of the car than if you loose grip on the front. Therefore it is best to put new tyres on the rear of the car to lessen the chance of loosing grip at the rear.

See this page which debunks the myth - “When replacing only two tires, the new ones go on the front”