Optimum rev range for fuel efficiency
Does anybody know what the optimal rev range is for getting the best fuel efficiency? For example:
In a 20 zone on the flat, I can choose 3rd gear at just over 1200rpm or 4th at just under 1000.
In a 40 zone I can choose 5th at just over 1300rpm or 6th at just under 1100.
My feeling from looking at the dis, is that in the 40 zone it is best to be in 6th but I'm unsure about the 20 zone. I have to stress that the road is dead flat and I'm not putting any excess load on the engine. It doesn't labour at just under 1000rpm in these conditions but I want to know if it would be better off going for the lower gear and slightly higher revs?