Slower aircrafts ?

Date:         26 Jun 98 02:37:57 
From:         "Frank Muenker" <frank@identity.de>
Organization: U Saarbruecken, Computational linguistics dept.
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Hi,

I was recently on a Delta B767-ER (-200 I think) flight from Frankfurt to
Cincinatti and the flight time was almost 10 hours. I could see on the
monitors that the average cruise speed was only about 480 mph which is
fairly slow compared to the 550 - 570 it could go. Also on 2 other
inner-European flights ( about 4.5 hrs) with a B737 and a B757 cruise speed
was way below 500 mph. I can recall that about 5 years ago cruise speed was
always 520-570, at least on flights longer than 1 hr.
I assume it has to do with fuel consumption, but does anybody know if the
fuel performance is significantly better at lower speeds and if so why did
they just start *now* to do that ? All aircrafts I was on looked like the
newest models. Is it that  the latest models have a different curve of fuel
performance ?

Any hints appreciated,

Frank