TrueCar.com, a publisher of information on car buying, says most vehicles reach peak fuel efficiency at speeds between between 45 and 55 miles per hour. Citing Energy Department figures, TrueCar says that for every five miles per hour in excess of 60 m.p.h., you lose 7 to 23 percent of fuel efficiency. Some commuters might save a gallon of gas a week this way.

Newscom
Besides being the law on this stretch of road, 55 miles per hour is a useful rule of thumb for good fuel economy.