I once owned a Rover 60, which would return over 30 m.p.g. when in free-wheel. This can’t be bad from a car weighing more than 3,000 lbs. I understand that Rover’s dropped this device when they introduced power brakes, as running out of petrol caused the engine to stop, followed by a loss of vacuum.
But for your “bread and butter” cars, may I suggest that Borg and Beck produce clutch friction plates with a simple free-wheel unit interposed between the driving splines and the friction surfaces. This should have the advantage of being cheap, easy to fit without modifications to the gearbox, usable in both forward and reverse gears and subject only to engine torque and not first gear torque. (This would also seem to be the correct place for an overdrive.) However, once installed, the driver would not have the choice of— “to free or not to free”. Personally, I would plump for the economy and reduction of stress in stop-start traffic, of free-wheeling, every time.
Llanelli John Williams