In my Cirrus SR22 the best glide speed is 87 or 88 knots indicated air-speed, depending on weight.
I don't doubt that I can get great gas economy by flying at that speed, but doesn't it miss the point?
The key term is "indicated airspeed". If one climbs at a constant power setting and a constant indicated airspeed, the true airspeed will increase with altitude. Therefore, if you use your 88 knots indicated plus 5 knots per Langeweiche's suggestion as the target for a given power setting, if you are indicating more than 93 knots in level flight, you will climb until your indicated airspeed (in level flight) is 93 knots as your true airspeed will increase with altitude at a fixed indicated airspeed.
Langewieshe wrote Stick and Rudder when a typical light plane was a Cub, a Luscombe or perhaps an Aeronca Chief. Many older airplanes lifted off at around 45 mph, climbed and glided between 55-60 and cruised between 80 and 90. The point is, the difference between best glide speed and best miles-per-gallon speed was only a few mph.
A modern aircraft like the SR-22 glides at about 85-90 knots and its best range would come somewhere around 50-55 percent power, or perhaps about 12o knots. Difference: 30-35 knots.
Don't get me wrong, I still go back and flip through Stick and Rudder now and then. That was an era when pilots flew by the seat of their pants, so unlike modern planes full of bells and whistles. The farther we get from 1942 the less relevant Langewishe's best endurance speed gets. And Mr. langeweishe's rule of thumb does not consider wind direction or how heavily loaded the aircraft is.
Best basic flying tip ever give to me: "Use your eyes to tell your hands to control direction, attitude and power",from an instructor during early dual instruction. Every control change we make in flying is either altering direction, changing roll/pitch/yaw or increasing/decreasing power.
I'm just finishing this book now, after having it recommended to me. I'm a student pilot, and this book offered, hands down, the best explanation of AoA that I've seen yet (and I do a LOT of reading).
The other significant (to me) education I rec'd by reading this book was the function and usage of the rudder. The book does a great job of identifying the pitfalls of poor rudder technique at the edges of the envelope. Very useful information.
Highly recommended, at least from the SP perspective!