Most drivers know that it is a good idea to periodically inspect a vehicle's tires to make sure they are in good condition. In addition to balancing, rotating, and aligning the tires, drivers can check the air pressure to make sure the tires aren't underinflated.

When it comes to replacing tires, drivers typically go by tread depth. The famous "penny test" has long been a standard measure in this area, assuring drivers that a tire is safe as long as the top of Abraham Lincoln's head isn't visible when the coin is inserted into a tread.

However, a tire's tread is only one way to determine its condition. You'll also want to make sure it isn't showing signs of dry rot, which increases the possibility of tread separation or a blowout.

Dry rot is the result of the gradual breakdown of the materials in the tire. Ben Wojdyla, writing for Popular Mechanics, says the oils used in synthetic rubber will evaporate over time. This process causes the chemical bonds used in the tire to start to break down. Ultraviolet radiation can also hasten the process. The AGCO Automotive Corporation, a repair shop in Baton Rouge, La., says additives in tires will protect against this degradation, but they too will break down over time.

A dry rotted tire is not as strong as a newer tire, and Wojdyla says the rubber will become brittle and lose its flexibility. The weakened tire will eventually fail, causing a blowout or tread separation. Angie's List says this failure is more likely to occur during highway driving, since this will produce more heat and cause the rubber to expand. However, it can also be dangerous to drive on a dry rotted tire at slower speeds as well.

Certain conditions or driving habits can make dry rot more likely to occur. Driving on underinflated tires can hasten the process, and degradation may occur more quickly if the tires are frequently exposed to sunlight or hot, dry conditions.

Tires are more likely to have dry rot if they are infrequently used, so it can be a more common occurrence on trailers and rarely driven classic cars. Doug DeMuro, writing for Autotrader, says vehicles that are left in one place for a long time can also develop flat spots, which will make the ride less comfortable.

The easiest way to spot dry rot is to look for small cracks in the rubber. If you spot dry rot early enough, you may be able to apply a water-based product to help seal the cracks. If it is more advanced, it is better to simply replace the tire, even if the tread depth is still sufficient.

Tires typically have a recommended expiration date, since dry rot is more likely to occur after the tire reaches a certain age. Rich Ellis, writing for the vehicle ownership site DriverSide, says many manufacturers recommend replacing tires after six or seven years. Wojdyla says tires generally should not be used for more than 10 years.

The easiest way to check the age of your tires is to look for a four digit code on the sidewall. This number refers to the date of the tire's manufacture, with the first two digits noting the week it was made and the last two digits denoting the year. So a figure of 2814 would indicate that the tire was made in the 28th week of 2014.

If a vehicle will remain in one place for a long time, you can take steps to avoid dry rot. Angie's List says it is best to store a vehicle in a climate-controlled garage with boards under the tires. Check the air pressure in the tires at least once every month and keep them inflated to the recommended levels.

You should also be aware of dry rot if you are purchasing used tires. The AGCO Automotive Corporation says that even if the tread looks sufficient, the tires might be expired or rotted.