Massachusetts-based Safety Research & Strategies recently sent a request to the National Highway Traffic Safety Administration to require "born-on" dates for passenger car tires and included an analysis of 50 crashes involving older tires. SRS President Sean Kane said rubber can begin to degrade after six years even if the tires have never been used.

It turns out most passenger vehicle tires already have a date stamped on them. The date is a small tag placed in the mold and it is formed into the tire sidewall when cured. These usually have the week of manufacture and the year (although some manufacturers might use weeks based on a fiscal year).

We called on the Firestone Ag Tire factory in DesMoines, IA, to find out if tractor tires are handled in a similar way. Here is engineer Jay Wheeler's response:

Firestone stamps manufacture dates on farm tires too (right now we are in week 45 so you will see the number 4504). We also code the tires with the particular curing press and side, if dual cavity, but that means nothing to the consumer.

All rubber is subject to ozone depletion, and that produces cracking. Any old tractor tire will show this. Part of what happens is when a tire is new the protective oils in the rubber work out to the surface to ward off the ozone. On a tractor (or any tire) that stands idle in the sun long enough, this gets worn away and the degradation is accelerated.

Rubber does harden up over time. One of our field engineers notes a higher-than-normal adjustments rate for farmers who take their brand new tires out to "bush hog" a rough piece of ground. The supple new rubber is more sensitive to puncture and damage than its grizzled old counterpart.