Not all GPS gear is created equal.  The maximum voltage that you may safely apply to the external DC power input (cigar lighter plug) varies from unit to unit. This is to answer the question for people who wanted to know if they could operate their GPS in their 24 or 48 volt electric automobiles and golf carts.

Here are the manufacturer's ratings for some popular units.

Garmin 
StreetPilot 26XX:  10-24 V
StreetPilot27XX:  10-18 V
StreetPilot7XXX:  10-18 V
Nüvi: 12-14 V
GPS 60Cx: 8-36 V
eTrex x Series:  8-36 V

NOTE:  The above voltage ratings are for the CIGAR LIGHTER PLUG and not the terminals on the unit itself.  Some units (such as eTrex, eMap, Rino) have the voltage regulator in the cigar lighter plug.  If you apply more than about 3 volts to the GPS unit itself, you will destroy it!

For the earlier Garmin handhelds you should be OK with 10 to 32 volts DC.