I asked this question of our tech folks regarding 757/767's. Digging in a Boeing manual we found 767-200 = 17.9:1 glide ratio.
757 has a better glide profile but that's with engines at idle. Assuming the dead engines on the 757 and 767 create the same amount of drag the 757 should glide better, especially now that they can have winglets installed.
Dual engine flameout checklist lists the best glide speed. It varies with weight. 757/767 a quick rule of thumb is Vref flaps 30 +95 kts. Not perfect, but very, very close for typical weights. Ranges from mid 200's to low 200's KIAS based on weight.
18:1 glide ratio is a great number. That glide ratio allows you to glide 3 miles for every 1,000' of altitude.
Eg, FL300, glide 90 n.m. 30,000/6000' per n.m. = 5 n.m. high. 5 n.m. x 18 = 90 n.m