Unless you've got some electrical connections at the transmission, you're not "in overdrive". The car is just able to freewheel because the planetary gear in the OD is not locked out. Unless you set the parking brake or park in Reverse the vehicle can roll away, even if it's parked in gear. If you want to lock out the OD (and eliminate the freewheeling) pull the lever on the side of the tail housing forward against the spring and lock it into position with a piece of heavy wire or a few strong zip ties. It's possible to make the OD fully functional with a fairly simple bit of wiring, a micro switch at the carburetor and a Bosch style relay. You don't necessarily need to have the OEM switch and relay.
Jerry
