If your tread is low on all 4 tires, it is wise to replace all 4 at the same time. For budget purposes you can replace 2 tires at a time.
〉 Answered on Jul 26th, 2008 by Amanda J. Pierce, MBA "AJ", Certified Mechanic, Project Coordinator; Primavera Scheduler at VIGOR Alaska Industrial Shipyard & BizzM3ch Solutions
If you are only replacing 2 tires, the best tires are always placed on the front for a front wheel drive vehicle.
It's best to replace them in pairs. If you only need one and the other is fairly new, then you will probably be okay. You want to have the tread thickness be as close as possible on the tires that are on the same axle.
Here's a great site to read more about tires.
〉 Answered on Jul 26th, 2008 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines