In September, all new cars sold in the U.S. will be required to have tire pressure monitoring systems. But for all those cars built without them, owners must rely either on their trusty gauge, or one of the new valve-stem monitors. If you've been wondering how well these things work, Consumer Reports has an answer for you.

They bought a set of Air Alert Valve Caps for $25 and put 'em to the test. These caps indicate a four pound drop in pressure by lighting up an LED, a pretty good idea.

You can read for yourself all the pros and cons, but the overall verdict was that they worked, but CR was concerned about durability and accuracy due to temperature-related fluctuations in tire pressure.

[Source: Consumer Reports]