well with the gps tracker , it sends you a message if the car gets out of 500 meters range, so even if your car is in a tow truck it will let you know where is it, and where are they going.
this is the one im using. some specs
Here is a small list of what it can do for you and your vehicle:
Track on Demand - Replies with a TXT message of current latitude, longitude, speed, and time, as well as an Internet map link to your cellphone to show you exactly where the GPS tracker/your vehicle is.
Geo-fencing control - When the vehicle moves in/out the preset scope, it will send an SMS alarm to the authorized phone numbers.
Movement and Speed Alert: The tracker can warm you with SMS message when the vehicle starts to move from a stationary status, or when the vehicle is moving faster than a preset speed.
Stop the Engine: You can send an SMS message to the tracker to cut off the gas and battery power, so the engine will remain in an immobilized state until your remove the alarm status.
http://www.chinavasion.com/product_info.php/pName/gps-car-tracker/