Does driving 10 mph slower really make THAT big a difference?
The reason is as clear as the air around you.
When cruising on the highway, your car will be in its highest gear with the engine humming along at relatively low rpm's. All your car needs to do is maintain its speed by overcoming the combined friction of its own moving parts, the tires on the road surface and, most of all, the air flowing around, over and under it.
Pushing the air around actually takes up about 40% of a car's energy at highway speeds, 
according to Roger Clark, a fuel economy engineer for General Motors. (cnn.money.com) 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.
Every 10 mph faster reduces fuel economy by about 4 mpg, a figure that remains fairly constant regardless of vehicle size, Clark said. (It might seem that a larger vehicle, with more aerodynamic drag, would see more of an impact. But larger vehicles also tend to have larger, more powerful engines that can more easily cope with the added load.)
This translates into a gallon of gas going up $.54 for every 10 miles per hour over 60.
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