It is an wise old saying that, “the faster you hurry, the slower you go.”  [from The Third Chinese Patriarch of Zen].  Stress factors aside, driving too fast is a waste of gasoline. 

The optimal highway speed is different for each vehicle.  But the general scientific consensus is that going faster than 55 mph (88.5 kph) is wasteful of gasoline.  Here’s a link to Science NetLinks on the topic of fuel efficiency.

Of course, it is safer to drive 55 mph or under, both for yourself and others.