A theft prevention system for an automobile having a power door

A theft prevention system (1) for an automobile includes an alarm system module (4) having: a logic circuit; an actual vehicle speed value input configured to receive an actual vehicle speed value from the automobile; a state selection input configured to receive a state selection instruction from a user; and a synthetic vehicle speed value output configured to output a synthetic vehicle speed value to the automobile. The logic circuit is configured so that, when the alarm system module (4) is in the disarmed state, the synthetic vehicle speed value is approximately equal to the actual vehicle speed value, and when the alarm system module (4) is in the armed state, the synthetic vehicle speed value is greater than a threshold value of approximately 2.5 km/hr.