I’m really liking the Wonderful World of Flying videos. This one starts with Emergency Glide Techniques with Barry Schiff.
In the video he talks about how you can’t extend the glide by varying from the best glide speed but that best glide speed does vary with weight.
I used his formula to adjust my weight and balance calculations for a Cessna T210L and his formula matches exactly with the speeds in the Owner’s Manual. You can use my on-line calculator to experiment.
You can also get the glide ratio from this graph. Just pick a point where the line is easy to read on both axes and then divide. In this case the line isn’t drawn precisely, but 20,000′ is 35 miles. So (35 miles * 5280 ft/mi) ÷ 20,000 feet = 9.25. And (25 miles * 5280 ft/mi) ÷ 14,000 feet = 9.4. So the glide ration is something like 9.3:1. I normally fly at around 5,500′ so I can glide around 9 miles.
Put another way, if you lose your engine in the pattern at 1,000′ AGL you can glide 9,000′ or just under 2 miles. For context, the runway at my field is 6,000′ so at pattern altitude I should be able to glide the length of the runway. The glide range will decrease if you turn.