Auto Tire Question

Hi,

Our older son borrowed my car (Subaru Outback wagon) for a few days to do some furniture moving. I did my usual minor maintenance check of his car (a 2000 Nissan Maxima), checked the fluids, and looked at the tires for unusual wear ...and began to wonder.

The depth of the tread is still OK...between 4/32 and 6/32 for each. However, I thought they might be getting "long in the tooth". Also, he has remarked to me that one of the tires seems to slowly leak air- goes down about 5 pounds over two weeks. He had the valve stem replaced but it still leaks slowly. He just fills it up about every two weeks . So, I checked when they were purchased: He bought and had mounted 4 Toyo Proxcess tires in March, 2007. At the time, they were supposed to be good for 40,000 miles.

Well, that is certainly a bit of an understatement. He now has 57,000 miles on them!

And so the question: The tires are about 5 1/3 years old and have 57,000 miles. My inclination is to do a "preventive" replacement, especially since one seems to be leaking slowly. Does anyone know of guidelines on how long one can use tires before replacing them?

Thanks

Susan.