Given the option of contesting a traffic ticket, most motorists–19 out of 20 by some estimates–would rather pay up than pit their word against a police officer’s in court. A retired sheriff’s deputy nevertheless hopes to beat the long odds of the law by setting the performance of a police officer’s radar gun against the accuracy of the GPS tracking device he installed in his teenage stepson’s car.

The log he downloaded, using software provided by the GPS unit’s Colorado-based supplier, showed the teen was going the speed limit within 100 feet of where a Petaluma officer clocked him speeding.