So what would an automotive engineer say about old tires?

"...tires will crack and become dry-rotted long before they wear out. This will occur faster when exposed to the UV effect of the sun. Since your (car) is under a carport, it is very possible that the tires on one side are baking in direct sunlight for a few hours a day. Those tires will deteriorate first.

"What I like to do is to visually inspect the sidewalls and in between the treads for cracking. I also use my finger to depress the tread to see how pliable it is. A new tire will be very soft, while an aged one will be very hard and not depress. You will eventually notice this in the ride quality and level of tire noise.

"At eight years, I would suggest a new set of tires. You may want to invest in tire covers like those used on recreational vehicles to protect the new tires from direct sunlight and the impact it has on the rubber."

This article by Ray Bohacz originally appeared in the September, 2008 issue of Hemmings Muscle Machines, archived on the web here:
http://www.hemmings.com/search_content/?0=0&bylineFacet=Ray Bohacz&sort=display_date_desc&page_size=15&start=150

Get out those fingers and depress.