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Quantity and quality of maps is critical.Furthermore, they must be current (or updateable), unless you will only drive in extremely mature cities such as New York or San Francisco. Some units can upload maps via a USB cable or a memory card. Internet-connected units from Garmin and TomTom can receive wireless, automatic updates.
Automatic destination routing is a standard feature on any good car navigation GPS. With more sophisticated auto GPS models you can input several destinations and the unit will tell you the most efficient route to hit them all.
Look for spoken street names. Instead of just giving an instruction to "turn left in 100 feet," models with text-to-speech can pronounce the street name ("turn left in 100 feet on Main Street"), giving you better information and reducing the need to look at the screen.
Make sure the unit you select is capable of being mounted neatly and securely on your dashboard or windshield. Nearly all models come with the proper hardware. Experts point out that you must be able to mount the unit where it won't interfere with airbag deployment. All U.S. states now allow GPS units to be mounted on the windshield.
Consider multimedia features. Many car GPS units play MP3s and can store photos. Most models play through integrated speakers, and some also offer an FM transmitter to play through your car stereo. However, tests find that FM transmitters don't work very well, and some models cannot play music and navigate at the same time.
Decide if you want to get live traffic reports and alerts. FM/TMC, MSN Direct, Navteq, XM, Tele Atlas and Clear Channel offer live traffic information. Compatible GPS units use a built-in or external receiver -- or a Bluetooth-compatible phone -- to download this information and incorporate it into routing. Some Garmin nuvi models include a free lifetime traffic subscription; other GPS units usually require a separate yearly subscription. Check coverage, however. Each service is only available in a limited, though growing, number of major markets. MSN Direct service will cease on Jan. 1, 2012, due to a lack of subscriptions, Microsoft has announced.
Make sure the screen size is appropriate for your eyesight and vehicle space. Most older and budget GPS models have 3.5-inch displays. Some budget models and most pricier units have 4.3-inch displays. Garmin offers some units with 5-inch displays.
Consider an auto GPS unit with a large points-of-interest database (5 million or more). Points of interest include tourist attractions, hotels, restaurants, gas stations and ATMs. Some units allow you to download custom points of interest. Web-connected units offer unlimited searching via the Internet, rather than relying on a finite database.
Weigh convenience features. A touch screen is now standard. Many units simplify text entry by only allowing you to type letters or names in the database. As you start to type, these units may also give you options to select. A few units allow voice commands, but experts say the Garmin nuvi 885T and nuvi 855 are the only GPS units that truly allow full voice operation.