BOAC - if Flaps 40 is an increased Vref for tail clearence what speed was it increased from? It's already at 1.25 Vso, which is basically the lowest multiplier used(either 1.25 or 1.3 x Vso).
And the 767-300 has the lowest pitch attitude for a tail strike but it uses 1.25 Vso also. But the 757, which has a higher pitch attitude for a tail strike, uses 1.3 Vso.
It might be
to you but perhaps you can clarify the 767-300 vs. the 757 as well as the 737-800 F30 vs. F40 "increased speed" issue?