I distinctly remeber learning from these boards, in the past, that a plane's best glide speed is usually the same as it's climb speed.
I also specifically remember a chart Pyro(was it Pyro?) showed me, that the best glide speed for an unpowered P-51D, was 175mph.
So I guess it can be said that usually about 175~200mph speed is optimum glide speed for almost all planes in AH.
The best glider is of course, the Me163, which the airframe itself, is made as a glider in the first place, in 1942. That thing flies forever at about 150mph glide speed.
The second best, from what I've experienced, was the Ta152H-1. I remember running out of fuel at mere 5000ft altitude - nearest base was more than 17miles away. I landed it safely at the base.
Some planes, seem quite fishy - La-7s for instance, while it has been said it's glide was corrected, it still glides insanely well.
Reducing the RPM does seem to have a positive effect, although it could be that in some cases it might be placebo.