Just found on a techie forum that in a 747 in clean configuration (ie flaps and gear up) you should aim for V2+80kts as your best glide speed.
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A fully loaded 747 has a glide ratio of 15:1 meaning that it travels 15 feet horizontal for every foot of vertical drop. That means at 35,000 feet it could travel about 100 miles. The new 787 Dreamliner is around 20:1.
Without engine thrust, the 747 had a glide ratio of 15:1, meaning it can glide forward 15 kilometres for every kilometre it drops. After calculating the glide ratio, the crew realized that they had less than 30 minutes to regain power before they smashed into the ground.
Without engine thrust, a 747-200 has a glide ratio of roughly 15:1, meaning it can glide forward 15 kilometres for every kilometre it drops. The flight crew quickly determined that the aircraft was capable of gliding for 23 minutes and covering 91 nautical miles (169 km) from its flight level of 37,000 feet (11,000 m).