If the tires don't have the same tread, they are not the same tire. If not the same tire, they have a different rolling radius which makes the tires turn at a different speed and that can be a big problem. It is a know fact that you don't just change 2 tires at once, but need to replace all 4 with the same tires at the same time. I have know members who have not and had some major problems after a period of time. It is surprising how many members don't know this and end up replacing only 2 at a time. This has been discussed many times here, so a search will bring up several threads about this. Good luck.