[78bronzechariot]

I haven't ever driven on in a 60s truck, but I have driven a late 30s car with one. If they are like the older ones, they are centrifugally engaged. You pull the handle to "turn on" the overdrive then, you accelerate through first, second, and third then lift off the gas. Then the overrunning clutch disengages and allows the flyweights to engage a drum and engage overdrive. They usually won't have enough speed for the flyweights to engage until you are at 3rd gear road speeds. A cool side note is that with the overrunning clutch you can shift through second and third without the clutch before the o.d. engages. In the late thirties, an electric solenoid was added to allow a forced downshift out of overdrive. Before that, you had to downshift into "second-over" for passing and climbing steep grades, or slow down to the point where the flyweights would disengage.