If you work for a trucking company or have a control-freak boss who tracks your every move, you might consider getting a device that disables GPS tracking. They are small, inexpensive, and don't affect other users around you.
One way to become invisible to your trackers is to buy the GPS Tracker Defense device, a small unit that emits a GPS-jamming signal within a five-meter radius---enough to knock out any tracking signals in your immediate area without disabling anybody else's GPS. This product is difficult to find in the U.S., but it is widely available in the United Kingdom. It retails for about £130 and can be found at http://www.spywarehouse.co.uk.
Buy the device. When it arrives, plug it into your car's 12-volt lighter receptacle. Within a minute it will start to work by preventing the GPS unit attached to your car or cellphone from sending a signal to the satellite.
An alternative way to thwart trackers is to simply turn off the GPS. This is not always possible, though, because there are secret software programs and tracking devices that can be hidden on your phone or car. You might not have access to them or even know of their existence.