Bookie, as ever, I bow to your superior opinion.
Some time you must give us the benefit of your advice of what we should do if flying a twin in which we don’t know the best glide speed.
I gave you my advice Timothy and I thought you rejected it when you wrote:
All this theory trying to reach an exact magic number when there is a lovely line already painted on every twin’s ASI which, everyone is agreed, is close enough and will give a very good approximation to best glide range.
The exact answer is that best glide speed depends on the aircraft, and is not related simply to blue line speed, as the cases we have identified show. Empirically, a bit faster than blue line or Vy would seem to fit better. The operational answer is that it doesn’t matter as the curve is pretty flat around the optimum, so if you want to pick blue line because it’s on the ASI that’s fine, you won’t lose much, but it’s not “a good approximation”.
Didn’t I say this earlier?
Dave_Phillips wrote:
For anything that doesn’t have a prescribed speed, I would use Vy for range and Vx for maximum time in the air. The numbers aren’t quite the same as best glide but good enough for the quoted scenario.
PS, the steam gauge twins I fly ordinarily have a 5kt discrepancy between ASIs. That sort of adds more fuel to the approximation argument. 
Quite. I have three ASIs – two mechanical and one glass, all calibrated in the last eight weeks, and all telling markedly different stories
Actually, Dave, I wonder if it’s because we fly (pretty much) the same type and that it is the two pitots that are to blame?
Hmmm, I would expect the digital to agree with one of the mechanical.
The easy way to check if it is a pitot issue is to just swap the ASIs, or equivalent.
The digital is much closer to one than the other. It doesn’t really cause enough bother to investigate, to be honest.
Who knows, if I have a double engine failure and follow the Blue Line, there is a 50/50 chance it will work in my favour 
