If the circumstances of the same size before and after: the best just before and after the change, such as the first two in two, because if not the same about wear will cause the driving instability, easy deviation, the car is generally the first two rounds of change after two rounds, if you want to replace the tire two two changed.
Some cars are different, some cars do not have, there are cars, high-grade cars for comfort, motility, so the front wheel and rear tire size is not the same. The average economy car is not the same, basically the same time, generally run for some time to see the degree of friction of the tire, then you can change the tire four corners. Extended tire usage
The size is different, the left and right change meaning is not big, can only follow the new tire.
Front of the car's front wheel wear is more serious, after the drive of the car's rear wheel wear more serious - so, cross change will be deviation, but also more damage to the tires, four-wheel drive car wear is not exactly the same. So I suggest: the same size before two on the two, or directly before two or after two when at the same time. Smart car
