Global navigation works like this: A tiny computer in your car communicates with a satellite computer that can tell with great precision where you are on the globe. Key in your destination, and the system calculates directions that appear on the screen in your car, or you can opt for voice prompts.

The technology—originally created for the military—is astonishing. Unfortunately, it hasn't yet translated into astonishing rental car devices.

At Avis, the PowerNAV arrives in a bulky briefcase, which you must pick up (and later return) separately from the vehicle. You stick an antenna on your dashboard, plug into the lighter, and sink a hook into the air-conditioning vent, affixing to it a 7-inch-square screen. The installation is complicated and fussy, but the real problem is that the system is unreliable. It may or may not recognize the address—or even the town—where you're heading. (On a test run through Northern California, the system could not direct me to the Gold Country hamlet of Jamestown or to any but the major thoroughfares in Sonora.) Instructions can be vague, or they may send you down roads 30 minutes out of your way. The rental fee is $6.99 per day, with an optional $2.95 per day insurance. You'll probably want that insurance: The replacement cost of the unit is $3,000.

Hertz's NeverLost ($6 per day) comes conveniently preinstalled on the dashboard. But like the Avis machine, it fails to recognize certain destination addresses. And it is sometimes downright dangerous: Deep inside New York City's Holland Tunnel, in traffic, NeverLost told us to make a right turn.

Until the rental companies acquire more sophisticated and dependable navigation devices for their fleets, we recommend that you get directions the old-fashioned way: from a map.