That sounds like the manual tranny. When you reach about 50 - 60 mph in 4th gear, you press the OD switch on the gear shift and the OD will cut in. I like to use the clutch at the same time to reduce the stress on the OD. When you press the switch again, the OD drops out and you are back in 4th gear. Anytime you shift down from 4th, the OD will drop out automatically. You have to be in 4th for the OD to work.

If the tranny is the automatic, it has 3 speeds plus OD. There is a yellow light on the right side of the instrument panel which says "OD Off" when it is lit. A small button on the left side of the tranny gear selector knob controls this light and the OD. If you are driving and the OD light is OFF, the OD will cut in automatically at about 35 mph depending om how far you are pushing down on the accelerator pedal. The farther you push, like merging on a freeway, the higher the speed at which the OD comes on. When we use our '84 240 (a great car) we just leave the OD light off so the OD is ON (sounds backward but there it is). When going down a long hill with a 35 mph speed limit, I use the switch to drop out of OD into 3rd to hold my speed down better.