Im not overly sure how it does it, but this was on the garmin site...
Unlike the 2610, the StreetPilot 2650 has dead reckoning capabilities, so you will continue to get navigation guidance even when GPS signals are obscured. To use this feature, a special dead reckoning cable must be connected to your vehicle's speedometer and backup lights by an authorized Garmin installer. Once installed, your StreetPilot will acknowledge your turns as well as your distance traveled when GPS reception is unavailable. Driving with dead reckoning capabilities is so seamless, you won't even know if you've lost GPS reception.