COLUMBIA, Mo. -

Before you head out for Memorial Day travel, there's a hidden danger in your car you may not be aware of - your tire's age.

Experts say millions could be driving with tires that are too old to be safe, and most don't know it.

Darek Baker has worked at McKnight Tires for five years.  He said 25% of their business is old tires.

"I see around two people a day come in with tires that are too old to drive on and have no idea," Baker said.

This is dangerous because an old tire can crack, dry rot, or worse - blow on the highway, leading to a serious accident.

"Especially heading into the warmer months, the heat makes things worse for tires," Baker said.

So how old is too old?

Tire dealers and rubber manufacturers say you shouldn't drive on a tire more than six to ten years old - no matter how much you drive on it.

There's a simple way to tell how old your tire is.

On the side of your tires, near the inside ring, there is a string of numbers that starts with DOT.

Follow those numbers and letters until you get to a series of four numbers.

Those four numbers represent the week and year the tire was made. So a tire that says "1109" means it was made in the 11th week of 2009, or in March of 2009.