http://www.openclip.net/Benchmark/IntroducingVz.pdf
Has anybody read this?
I have skimmed it – it is dense and a close study would take a number of days – and would make two observations
Firstly I don’t think it is very relevant to aircraft which are CHT limited in climb, and in which you climb using the constant-EGT method and adjust the VS to maintain a CHT below say 400F – like the TB20.
Secondly I think this is a good summary – page 42
On a quick look at the graphs, the actual speeds are not all that critical, so say a 120kt climb in the TB20 (anything less drives the CHT too high) seems reasonable.
I have done measurements of climb fuel burn in the past, using different techniques, and while the time taken to reach a given altitude obviously vary, the amount of fuel burnt to reach it are not different enough to worry about it. I doubt that the absolute optimal technique (whatever it is) would save more than 1-2 USG over doing it quite crudely.
But that is the case for a relatively high-perf aircraft; if you were trying to climb to FL140 in a PA28-181 and then your zero-fuel range was say 600nm and you were trying to fly 500nm, it may be different. But then unless your PA28-181 has a fuel totaliser the saving will be way smaller than the normal uncertainly in the fuel burn anyway…
380F in climb… you would be lucky 
For all the words, I was rather disappointed by the very light description of why Vz is 1.32 Vy.
Unless I’m misunderstanding, Vz is based on Carson’s paper and a fairly arbitrary definition of efficiency as the speed divided by the usual fuel per unit distance, on the basis that minimising the fuel per unit distance alone is boringly slow. If you’re wondering where 1.32 comes from, it’s the fourth root of three (3^0.25), the same ratio as the best glide range speed to the minimum sink rate speed in a glide, for similar reasons.
It seems to make the assumption that best L/D (i.e. minimum energy required per unit distance) is Vy. While it might be a decent rule of thumb, there’s no reason to assume that the optimum excess power occurs at best L/D.
On a tangent, I also think that Vbg is not the best-MPG speed under power – because engine efficiency is poor at the low power settings require to fly at Vbg…
It’s fine to speak of prop efficiency in great detail but what about engine efficiency?
