Best time to use 6th gear?

Having driven with a 5 speed gearbox all my life, I recently changed to a car with 6 forward gears. I have been wondering when is the best time to hit the 6th. I was on the M1 today and was travelling at a fairly constant speed of 100km/h in rush hour traffic. In 5th at this speed, the car hits about 2200RPM, while in 6th it reaches only 1800RPM. In terms of fuel economy, reducing stress on the engine etc., what is the best thing to do? And should 6th gear only be used on motorways at a constant high speed? Thanks

It's a cruising gear. Motorway/duel carriage way. I use it when using cruise control

Maybe there's more to it than this, but my approach is simply to drive in 6th at the speeds I would have been using 5th in - lower by about 500 rpm= better economy.

Is it any more complex than this?

if it's a rental car, the best way to change down is directly from 6th to reverse .... as opined by Jeremy 'my favourite car is a rental' Clarkson

Should be in the highest gear you can at all times "where" the engine isn't struggling. That should give you the lowest revs and max fuel ecomomy assuming drag at higher speeds isn't a factor, 100+