EclectEcon

The other day, Ms. Eclectic asked me what the best speed is to drive. As a good economist, I replied the same way all economists respond to all questions.
It all depends.
I said that I thought fuel usage declined the slower you went (turns out that was seriously wrong) but that it would take longer to reach our destination, so there would be a trade-off between our spending for gasoline, the risks of accidents and injuries at different speeds, and how much we value our time en route versus our time in its next best use.

My friend, Steve, says he used to do about 110 kmh all the time on highways (with speed limits of only 80kmh), but about two years ago he decided to reduce the risks of accidents and reduce his use of gasoline. He now rarely, if ever, goes over 90 kmh.

Most of the time on the highway I drive at speeds between 85 and 95 kmh (which, for the metricly challenged, is between about 53 and 60 mph). It turns out this speed is also roughly in the optimal range for fuel economy according to this item at Econobrowser. Here is a graph from that posting plotting average fuel mileage against speed for a sample of automobiles.
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Addendum: for more, see the ever-informative Political Calculations.