A fuel cap for a filler neck on a fuel tank includes a cap body having threads for engaging the filler neck when the fuel cap is installed on the filler neck and a cap handle rotatably mounted on the cap body. A clutch acts between the cap handle and the cap body to permit the cap handle to rotate freely on the cap body when the clutch is disengaged. When the clutch is engaged, rotation of the cap handle will rotate the cap body so that the user can remove the fuel cap from the filler neck to refuel the vehicle. A pressure-sensing diaphragm disengages the clutch when the pressure differential between the fuel vapor pressure and the atmospheric pressure exceeds a predetermined threshold and engages the clutch when the pressure differential between the fuel vapor pressure and the atmospheric pressure is less than the predetermined threshold.