How? Read Slow Down a Little, Save a Lot of Gas. Your car gets less efficient as you start going faster than 60:
"Traveling faster makes the job even harder. More air builds up in front of the vehicle, and the low pressure "hole" trailing behind gets bigger, too. Together, these create an increasing suction that tends to pull back harder and harder the faster you drive. The increase is actually exponential, meaning wind resistance rises much more steeply between 70 and 80 mph than it does between 50 and 60."
"Engineers at Consumer Reports magazine tested this theory by driving a Toyota Camry sedan and a Mercury Mountaineer SUV at various set cruising speeds on a stretch of flat highway. Driving the Camry at 75 mph instead of 65 dropped fuel economy from 35 mpg to 30."
That is about a 14% drop in efficiency. If you pay $3 per gallon, it costs you about 42 cents a gallon. The faster you go above 60 and/or the more you pay for gas, the bigger the decline in fuel economy. Truck drivers are now slowing down to save money. Read Drivers slow down their tractor-trailer rigs to save fuel.