Vehicle fuel theft prevention assembly

A theft prevention assembly for use on an externally threaded outer end of a fuel tank filling neck includes an outer cap, an internally threaded inner cap for mounting on the filling neck, and a lock assembly defined by a cup attached to the inner cap, a sleeve slidably in the cup and a lock in the sleeve. The outer cap is normally freely rotatable relative to the fixed inner cap, the cup and the sleeve which are releasably interconnected by the lock. When the cup is released from the sleeve, the inner cap is latched to the outer cap by the sleeve, and the two caps can be rotated as a unit to open the filling neck.